Cnt-based signature control material

ABSTRACT

A radar absorbing composite includes a (CNT)-infused fiber material disposed in at least a portion of a matrix material. The composite absorbs radar in a frequency range from about 0.10 Megahertz to about 60 Gigahertz. The CNT-infused fiber material forms a first layer that reduces radar reflectance and a second layer that dissipates the energy of the radar. A method of manufacturing this composite includes disposing a CNT-infused fiber material in a portion of a matrix material with a controlled orientation of the CNT-infused fiber material within the matrix material, and curing the matrix material. The composite can be formed into a panel which is adaptable as a structural component of a transport vessel or missile for use in stealth applications.

STATEMENT OF RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.61/173,435, filed Apr. 28, 2009, and U.S. Provisional Application No.61/172,503, filed Apr. 24, 2009, both of which are incorporated hereinby reference in their entirety.

FIELD OF INVENTION

The present invention relates to generally to radar absorbing materials.

BACKGROUND

Low observable, or stealth, technology is utilized on aircrafts, ships,submarines, and missiles, for example, to make them less visible orobservable to radar, infrared, sonar and other detection methods.Various radar absorbing materials (RAMs), which absorb electromagneticfrequencies, such as in the radar range, have been developed for suchlow observable applications. However, the RAMs presently employed havesome drawbacks. For example, many RAMs are not an integral part of thesurface of a low observable structure. Instead, the RAMs are applied ascoatings or paints over the surface of the low observable structuremaking them heavier, and prone to wear, chipping, and failure. Anexample of such a RAM includes iron ball paint, which contains tinyspheres coated with carbonyl iron or ferrite. Moreover, these coatingsrequire bonding to the surface of the structure because they are not anintegrated part of the structure or surface.

Another example of a RAM is urethane foam impregnated with carbon. SuchRAMs are used in very thick layers. Such RAMs are inherentlynon-structural in nature such that they add weight and volume tostructures while providing no structural support. These types of foamRAMs are frequently cut into long pyramids. For low frequency damping,the distance from base to tip of the pyramid structure is often 24inches, while high frequency panels can be as short as 3-4 inches.

Another RAM takes the form of doped polymer tiles bonded to the surfaceof the low observable structure. Such tiles which include neoprene dopedwith carbon black or iron particles, for example, are prone toseparation, particularly in extreme operating environments such asextremely high or low temperatures, and/or high altitudes. Finally,numerous RAMs do not perform adequately in the long radar wavelengthband, about 2 GHz.

It would be beneficial to develop alternative RAMs that address one ormore of the aforementioned issues. The present invention satisfies thisneed and provides related advantages as well.

SUMMARY OF THE INVENTION

In some aspects, embodiments disclosed herein relate to a radarabsorbing composite that includes a (CNT)-infused fiber materialdisposed in at least a portion of a matrix material. The composite iscapable of absorbing radar in a frequency range from between about 0.10Megahertz to about 60 Gigahertz. The CNT-infused fiber material forms afirst layer that reduces radar reflectance and a second layer thatdissipates the energy of the absorbed radar.

In some aspects, embodiments disclosed herein relate to a method ofmanufacturing a radar absorbing composite that includes a (CNT)-infusedfiber material disposed in at least a portion of a matrix material. Thecomposite is capable of absorbing radar in a frequency range frombetween about 0.10 Megahertz to about 60 Gigahertz. The CNT-infusedfiber material forms a first layer that reduces radar reflectance and asecond layer that dissipates the energy of the absorbed radar. Themethod includes disposing a CNT-infused fiber material in a portion of amatrix material with a controlled orientation of the CNT-infused fibermaterial within the matrix material, and curing the matrix material. Thecontrolled orientation of the CNT-infused fiber material controls therelative orientation of CNTs infused thereon.

In some aspects, embodiments disclosed herein relate to a panel thatincludes a composite including a (CNT)-infused fiber material disposedin at least a portion of a matrix material. The composite is capable ofabsorbing radar in a frequency range from between about 0.10 Megahertzto about 60 Gigahertz. The CNT-infused fiber material forms a firstlayer that reduces radar reflectance and a second layer that dissipatesthe energy of the absorbed radar. The panel is adaptable to interface asa structural component of a transport vessel or missile for use instealth applications.

In some aspects, embodiments disclosed herein relate to a transportvessel that includes the aforementioned composite in the form of apanel. The CNTs infused on the fiber material have a controlledorientation within the composite material.

In some aspects, embodiments disclosed herein relate to a projectilethat includes the aforementioned composite in the form of a panel. TheCNTs infused on the fiber material have a controlled orientation withinthe composite material.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a transmission electron microscope (TEM) image of amulti-walled CNT (MWNT) grown on AS4 carbon fiber via a continuous CVDprocess.

FIG. 2 shows a TEM image of a double-walled CNT (DWNT) grown on AS4carbon fiber via a continuous CVD process.

FIG. 3 shows a scanning electron microscope (SEM) image of CNTs growingfrom within the barrier coating where the CNT-forming nanoparticlecatalyst was mechanically infused to the carbon fiber material surface.

FIG. 4 shows a SEM image demonstrating the consistency in lengthdistribution of CNTs grown on a carbon fiber material to within 20% of atargeted length of about 40 microns.

FIG. 5 shows an SEM image demonstrating the effect of a barrier coatingon CNT growth. Dense, well aligned CNTs grew where barrier coating wasapplied and no CNTs grew where barrier coating was absent.

FIG. 6 shows a low magnification SEM of CNTs on carbon fiberdemonstrating the uniformity of CNT density across the fibers withinabout 10%.

FIG. 7 shows a cross-section of a radar absorbing composite materialhaving a carbon nanotube (CNT)-infused fiber material.

FIG. 8 shows a carbon nanotube-infused fiber tow adapted to be used as aradar absorbing material coating on an article such as a panel.

FIG. 9 shows a carbon nanotube-infused fiber tow coating applied on acomposite to improve the radar absorbing characteristics of thecomposite.

FIG. 10 shows a schematic diagram of a coating system for carbonnanotube-infused fibers.

FIG. 11 shows a process for producing CNT-infused carbon fiber materialin accordance with the illustrative embodiment of the present invention.

FIG. 12 shows how a carbon fiber material can be infused with CNTs in acontinuous process to target thermal and electrical conductivityimprovements for radar absorption.

FIG. 13 shows a cross section of an exemplary RAM panel having a bilayerstructure that includes CNT-infused fiber materials.

FIG. 14 shows a cross section of an exemplary RAM panel having amulti-layered structure that includes CNT-infused fiber materials.

DETAILED DESCRIPTION

The present invention is directed, in part, to composite materials thatare RAMs. The radar absorbing composite materials disclosed herein haveCNT-infused fiber materials disposed in a portion of a matrix material.CNTs have desirable electromagnetic absorption properties due to theirhigh aspect ratio, high conductivity, and when infused to a fibermaterial can be tailored for specific surface coverage densities. TheCNTs in the overall composite are capable of absorbing radar anddissipating the absorbed energy as heat, for example.

The radar absorbing composite materials of the invention can improve theabsorption characteristics of already low observable surfaces. In someembodiments, the CNT-infused fibers impart improved signature control ofdielectric (insolative—transparent to radar) as well as conductive(significantly reflective to radar) composite materials, resulting inthe ability to use low weight, high strength composites. Some suchcomposites may have been previously limited in application due to theirinherently poor signature control capabilities.

Radar absorbing composite materials of the invention can provide anabsorbent surface that is nearly a black body across different sectionsof the electromagnetic spectrum including the visible region and variousradar bands. CNTs infused on fibers allows for tailored arrangement ofparticular CNT densities in various layers to create a radar absorbingstructure. That is, the radar absorbing capacity can be achieved byproviding varying CNT density across the depth of the material. Thefiber material is a scaffold that organizes the CNTs in an array thatprovides an overall composite with appropriate CNT density at differentdepths to provide internal reflection in some layers and effectivepercolation pathways for dissipation of the energy upon radar absorptionin other layers. Still other layers can provide a combination ofinternal reflection and percolation pathways to dissipate the absorbedradar energy. The infused CNTs can be tailored to have a uniform length,density, and controlled orientation on the fiber material based on acontinuous CNT infusion process. The CNT-infused fiber thus obtained isthen disposed within a composite structure to maximize radar absorption.

In particular, near the surface of a composite, CNT densities can berelatively low creating a material that has a dielectric constantsimilar to air or a refractive index close to air creating a blackbody-like structure where radar reflectance is substantially minimized.That is, in order to suppress reflection, the refractive index of theobject can be close to that of air. This solution to minimizereflectance is evident from Fresnel's law:

R=(n−n ₀)²/(n+n ₀)²

where R is reflectance, n is the refractive index of the object, and n₀is the refractive index of air. The CNT density on the fiber materialcan be modulated in the continuous process described herein below suchthat the CNT-infused fiber material can be tuned to exhibit a CNTdensity such that the refractive index, n, in a layer of CNT-infusedfiber within a composite structure approximates that of air, n₀.

By relying on CNTs for radar absorption, the composite materials canutilize either conducting or insulating fiber materials and/or matrices.Moreover, the radar absorbing composite materials can be integrated aspart of the surface and/or the overall structure of the low observable.In some embodiments, the entire structure can function as a RAM,obviating the issues of wear, chipping and the like associated withcoated RAM paints, for example. Significantly, unlike the urethane-typefoams, the RAMS of the present invention are structural and thus,substantial weight reductions can be achieved relative to their foamcounterpart. In some embodiments, CNT-infused fiber materials can beemployed as a coating while avoiding the problems associated withchipping/wear, and the like due to the extended lengths of fibermaterial employed.

The manufacturing process to create CNT-infused fibers for theaforementioned radar absorbing materials is described herein furtherbelow. The process is amenable to large scale continuous processing. Inthe process, CNTs are grown directly on carbon, glass, ceramic, orsimilar fiber materials of spoolable dimensions, such as tows orrovings. The nature of the CNT growth is such that a dense forest isdeposited at lengths that can be tuned between about 100 nanometers toabout 500 microns long, the length being controlled by various factorsas described below. This forest can be oriented such that the CNTs areperpendicular to the surface of each individual filament of a fibermaterial thus providing radial coverage. In some embodiments, the CNTscan be further processed to provide an orientation that is parallel tothe axis of the fiber material. The resulting CNT-infused fibermaterials can be wound as manufactured or can be woven into fabric goodsfor use in producing the radar absorbing composite materials used in lowobservable structures. Significantly, the continuous process can allowfor the production of sections of CNT-infusion with varied CNT density.As explained further below, this readily allows for the manufacture ofmulti-layered structures which when assembled, contribute to the overallradar absorbing capability.

As used herein, the term “radar absorbing composite material” refers toany composite material that has at least a CNT-infused fiber materialdisposed in a matrix material. The radar absorbing composite materialsof the invention have three components, CNTs, a fiber material, and amatrix material, that create an organized hierarchy wherein the CNTs areorganized by the fiber material to which they are infused. TheCNT-infused fiber material is, in turn, organized by the matrix materialin which it is disposed. The CNTs, arranged in particular densitiesaccording to the depth of a given layer, can prevent radar reflectanceand/or absorb electromagnetic (EM) radiation associated with a radartransmitting source or reflected EM from an object in detectionapplications. The absorbed radar can be converted to heat and/or anelectrical signal.

As used herein, the term “radar” refers to any of the common bands ofradar frequencies ranging from about 0.10 Megahertz to about 60Gigahertz. Radar absorbing composite materials of the present inventionare particularly effective, for example, in the L- through K-band asdescribed herein further below.

As used herein, the term “radar absorption capacity” refers to theability of the radar absorbing composite materials of the presentinvention to absorb electromagnetic radiation of any radar band.

As used herein, the term “fiber material” refers to any material whichhas fiber as its elementary structural component. The term encompassesfibers, filaments, yarns, tows, tows, tapes, woven and non-wovenfabrics, plies, mats, 3D woven structures and the like.

As used herein, the term “spoolable dimensions” refers to fibermaterials having at least one dimension that is not limited in length,allowing for the material to be stored on a spool or mandrel. Fibermaterials of “spoolable dimensions” have at least one dimension thatindicates the use of either batch or continuous processing for CNTinfusion as described herein. Fiber materials of “spoolable dimensions”can be obtained commercially as glass, carbon, ceramic, and similarproducts. An exemplary carbon fiber material of spoolable dimensionsthat is commercially available is exemplified by AS4 12 k carbon fibertow with a tex value of 800 (1 tex=1 g/1,000 m) or 620 yard/lb (Grafil,Inc., Sacramento, Calif.). Commercial carbon fiber tow, in particular,can be obtained in 5, 10, 20, 50, and 100 lb. (for spools having highweight, usually a 3 k/12K tow) spools, for example, although largerspools may require special order. Processes of the invention operatereadily with 5 to 20 lb. spools, although larger spools are usable.Moreover, a pre-process operation can be incorporated that divides verylarge spoolable lengths, for example 100 lb. or more, into easy tohandle dimensions, such as two 50 lb spools.

As used herein, the term “carbon nanotube” (CNT, plural CNTs) refers toany of a number of cylindrically-shaped allotropes of carbon of thefullerene family including single-walled carbon nanotubes (SWNTs),double-walled carbon nanotubes (DWNTs), and multi-walled carbonnanotubes (MWNTs). CNTs can be capped by a fullerene-like structure oropen-ended. CNTs include those that encapsulate other materials.

As used herein “uniform in length” refers to the length of CNTs grown ina reactor. “Uniform length” means that the CNTs have lengths withtolerances of plus or minus about 20% of the total CNT length or less,for CNT lengths varying from between about 1 micron to about 500microns. At very short lengths, such as 1-4 microns, this error may bein a range from between about plus or minus 20% of the total CNT lengthup to about plus or minus 1 micron, that is, somewhat more than about20% of the total CNT length. In signature control (radar absorption) thelengths (as well as density of coverage) of the CNTs can be used tomodulate radar absorption and can be optimized for absorption maxima ina targeted radar band.

As used herein “uniform in distribution” refers to the consistency ofdensity of CNTs on a carbon fiber material. “Uniform distribution” meansthat the CNTs have a density on the carbon fiber material withtolerances of plus or minus about 10% coverage defined as the percentageof the surface area of the fiber covered by CNTs. This is equivalent to±1500 CNTsμm² for an 8 nm diameter CNT with 5 walls. Such a figureassumes the space inside the CNTs as fillable.

As used herein, the term “infused” means bonded and “infusion” means theprocess of bonding. Such bonding can involve direct covalent bonding,ionic bonding, pi-pi, and/or van der Waals force-mediated physisorption.For example, in some embodiments, the CNTs can be directely bonded tothe carbon fiber material. Bonding can be indirect, such as the CNTinfusion to the carbon fiber material via a barrier coating and/or anintervening transition metal nanoparticle disposed between the CNTs andcarbon fiber material. In the CNT-infused carbon fiber materialsdisclosed herein, the carbon nanotubes can be “infused” to the carbonfiber material directly or indirectly as described above. The particularmanner in which a CNT is “infused” to a carbon fiber materials isreferred to as a “bonding motif.”

As used herein, the term “transition metal” refers to any element oralloy of elements in the d-block of the periodic table. The term“transition metal” also includes salt forms of the base transition metalelement such as oxides, carbides, nitrides, and the like.

As used herein, the term “nanoparticle” or NP (plural NPs), orgrammatical equivalents thereof refers to particles sized between about0.1 to about 100 nanometers in equivalent spherical diameter, althoughthe NPs need not be spherical in shape. Transition metal NPs, inparticular, serve as catalysts for CNT growth on the carbon fibermaterials.

As used herein, the term “sizing agent,” “fiber sizing agent,” or just“sizing,” refers collectively to materials used in the manufacture ofcarbon fibers as a coating to protect the integrity of carbon fibers,provide enhanced interfacial interactions between a carbon fiber and amatrix material in a composite, and/or alter and/or enhance particularphysical properties of a carbon fiber. In some embodiments, CNTs infusedto carbon fiber materials behave as a sizing agent.

As used herein, the term “matrix material” refers to a bulk materialthan can serve to organize sized CNT-infused fiber materials inparticular orientations, including random orientation. The matrixmaterial can benefit from the presence of the CNT-infused fiber materialby imparting some aspects of the physical and/or chemical properties ofthe CNT-infused fiber material to the matrix material. In radarabsorption applications, the matrix material in conjunction fibermaterial, provide controlled CNT densities and controlled CNTorientation. Such control is far more difficult to achieve by simplemixing of loose CNTs with the matrix alone. The greater control ofdensities along the CNT-infused fiber material provides a means to formlayers that prevent reflectance of radar and layer that providepercolation pathways to dissipate the absorbed radar energy. The RAMs ofthe present invention typically have higher density of CNTs in lower plylayers that at the surface.

As used herein, the term “material residence time” refers to the amountof time a discrete point along a fiber material of spoolable dimensionsis exposed to CNT growth conditions during the CNT infusion processesdescribed herein. This definition includes the residence time whenemploying multiple CNT growth chambers.

As used herein, the term “linespeed” refers to the speed at which afiber material of spoolable dimensions can be fed through the CNTinfusion processes described herein, where linespeed is a velocitydetermined by dividing CNT chamber(s) length by the material residencetime.

In some embodiments, the present invention provides a radar absorbingcomposite material that includes a (CNT)-infused fiber material disposedin at least a portion of a matrix material. The composite material iscapable of absorbing radar in a frequency range from between about 0.1MHz to about 60 GHz. 1. The CNT-infused fiber material can lay up in thecomposite to form a first layer that reduces radar reflectance, that is,transmits incident radar, and a second layer that dissipates the energyof the absorbed radar. The first layer can be of a thickness to optimizeinternal reflection utilizing the quarter wavelength rule. The secondlayer can aid in dissipating energy by at least two mechanisms. Theimpinging radar can be absorbed, in part, and the energy converted toelectrical or heat energy. The impinging radar can also be reflectedback to the first layer which is internally reflective, resulting indissipation of the radar energy as heat.

In some embodiments, composites of the invention can include a pluralityof additional layers between the first layer and second layer. Theseintermediate layers can be provided as a stepped gradient of increasingCNT density on the CNT-infused fiber material from the first layer downto the second layer. In other embodiments, these intermediate layers canbe provided as a continuous gradient of increasing CNT density on theCNT-infused fiber material from the first layer down to the secondlayer.

In some embodiments, the first layer and the second layer can beseparate CNT-infused fiber materials. That is separate lengths of twocontinuous fiber materials. This provides diversity of potentialcomposition for the fiber material. For example, in some embodiments,the first layer can include a CNT-infused glass fiber material, while inother embodiments, the second layer can include a CNT-infused carbonfiber material. The inner most layers of the composite structure canbenefit from utilizing a fiber material that has its own conductingproperties. This can aid in dissipation of the absorbed radar energy.

One skilled in the art will recognize that in applications related toradar absorption for signature control, it is desirable to manufacturematerials that absorb and/or transmit radar while avoiding radarreflection at the surface of the object. From a mechanistic standpoint,radar absorbing applications benefit from the absorption characteristicsprovided by the presence of the CNT-infused fiber material and theability to minimize radar reflectance at lower CNT densities.

The radar absorbing composite materials include CNT-infused fibermaterials that are typically constructed by infusing CNTs on“continuous” or “spoolable” lengths of a fiber material such as a tow,roving, fabric, or the like. The radar absorption capacity can varydepending on, for example, CNT length, CNT density, and CNT orientation.The processes by which CNT-infused fiber materials are made allow forthe construction of radar absorbing composites with well-definedabsorption capability. The CNT length and orientation on the fibermaterial is controlled in the CNT growth process described herein below.The relative orientation of the CNTs in the composite is in turncontrolled by the composite manufacturing process which orients theCNT-infused fiber.

The radar absorbing composite materials of the invention can beconstructed to absorb one or more radar bands. In some embodiments, asingle spoolable length of CNT-infused fiber can be provided that hasdiffering lengths and orientations of CNTs along different sections ofthe single spoolable length in order to maximize absorption of differentradar frequency bands. Alternatively, multiple spoolable lengths offiber material with differing CNT lengths and/or orientations can bedisposed in the composite material for the same effect. Either strategyprovides layers within a composite with differing radar absorptioncharacteristics. The multiple orientations for the CNTs also allow theradar absorbing composite to absorb electromagnetic radiation frommultiple radar sources impinging at different incident angles on thecomposite material.

The dense packing of CNTs in the second or innermost layers of thecomposite can provide percolation pathways to effectively dissipate theenergy of the absorbed radar electromagnetic radiation. Without beingbound by theory, this can be the result of CNT-to-CNT point contact orCNT-to-CNT interdigitation as exemplified in FIGS. 7-9. In someembodiments, the absorbed radar energy in the CNTs can be transformedinto electrical signals that can be integrated with a computer system tomodulate the orientation of an article that incorporates the radarabsorbing composite, such as a panel, to maximize radar absorption inresponse to a radar transmitting source or in a reflected EM wave indetection applications, for example. The second or innermost layers canalso internally reflect the radar and dissipate the energy as heat.

In some embodiments, the radar absorbing composite material is providedas integral part of an entire article or structure used in stealthapplications. In other embodiments, the radar absorbing material can beprovided in a portion of the overall composite structure. For example acomposite structure can have a surface “skin” that incorporatesCNT-infused fiber material to absorb radar. In other embodiments, theradar absorbing composite material can be applied as a coating, forexample as a chopped fiber mixed in coating matrices, on an alreadyexisting surface of another composite or other article. In suchembodiments, the coating employs long lengths of fiber material whichhelps prevent chipping and the like as might occur with conventionalcoatings. Moreover, when employed as a coating a radar transparentovercoating can be used to further protect the radar absorbing compositematerial. Also when used as a “coating” the matrix of the CNT-infusedfiber composite can closely match or be identical to the bulk matrix ofthe overall structure to provide superior bonding.

CNT-infused fiber materials of the radar absorbing composite materialsare provided in which the CNTs are substantially uniform in length. Thisprovides an overall composite product with reliable absorptionproperties across large sections. In the continuous process describedherein for the production of CNT-infused fiber materials, the residencetime of the fiber material in a CNT growth chamber can be modulated tocontrol CNT growth and ultimately, CNT length. This provides a means tocontrol specific properties of the CNTs grown. CNT length can also becontrolled through modulation of the carbon feedstock and carrier gasflow rates and reaction temperature. Additional control of the CNTproperties can be obtained by controlling, for example, the size of thecatalyst used to prepare the CNTs. For example, 1 nm transition metalnanoparticle catalysts can be used to provide SWNTs in particular.Larger catalysts can be used to prepare predominantly MWNTs.

Additionally, the CNT growth processes employed are useful for providinga CNT-infused fiber material with uniformly distributed CNTs on thefiber materials while avoiding bundling and/or aggregation of the CNTsthat can occur in processes in which pre-formed CNTs are suspended ordispersed in a solvent solution and applied by hand to the fibermaterial. Such aggregated CNTs tend to adhere weakly to a fiber materialand the characteristic CNT properties are weakly expressed, if at all.In some embodiments, the maximum distribution density, expressed aspercent coverage, that is, the surface area of fiber covered, can be ashigh as about 55% assuming about 8 nm diameter CNTs with 5 walls. Thiscoverage is calculated by considering the space inside the CNTs as being“fillable” space. Various distribution/density values can be achieved byvarying catalyst dispersion on the surface as well as controlling gascomposition and process speed. Typically for a given set of parameters,a percent coverage within about 10% can be achieved across a fibersurface. Higher density and shorter CNTs are useful for improvingmechanical properties, while longer CNTs with lower density are usefulfor improving thermal and electrical properties and radar absorption,although increased density is still favorable. A lower density canresult when longer CNTs are grown. This can be the result of the highertemperatures and more rapid growth causing lower catalyst particleyields.

CNTs useful for infusion to the fiber materials include single-walledCNTs, double-walled CNTs, multi-walled CNTs, and mixtures thereof. Theexact CNTs to be used depends on the end-use application of the radarabsorbing composite material. CNTs can be used for thermal and/orelectrical conductivity applications, in addition to radar absorption.In some embodiments, the infused carbon nanotubes are single-wallnanotubes. In some embodiments, the infused carbon nanotubes aremulti-wall nanotubes. In some embodiments, the infused carbon nanotubesare a combination of single-wall and multi-wall nanotubes. There aresome differences in the characteristic properties of single-wall andmulti-wall nanotubes that, for some end uses of the fiber, dictate thesynthesis of one or the other type of nanotube. For example,single-walled nanotubes can be semi-conducting or metallic, whilemulti-walled nanotubes are metallic. Thus, if it can be desirable tocontrol the CNT type if the absorbed radar is to be converted into, forexample, and electrical signal that can integrate with a computersystem.

CNTs lend their characteristic properties such as mechanical strength,low to moderate electrical resistivity, high thermal conductivity, andthe like to the CNT-infused fiber material. For example, in someembodiments, the electrical resistivity of a carbon nanotube-infusedcarbon fiber material is lower than the electrical resistivity of aparent fiber material. More generally, the extent to which the resultingCNT-infused fiber expresses these characteristics can be a function ofthe extent and density of coverage of the carbon fiber by the carbonnanotubes. These properties can also be transferred to the overall radarabsorbing composite in which they are incorporated. Any amount of thefiber surface area, from 0-55% of the fiber can be covered assuming an 8nm diameter, 5-walled MWNT (again this calculation counts the spaceinside the CNTs as finable). This number is lower for smaller diameterCNTs and more for greater diameter CNTs. 55% surface area coverage isequivalent to about 15,000 CNTs/micron². Further CNT properties can beimparted to the carbon fiber material in a manner dependent on CNTlength. Infused CNTs can vary in length ranging from between about 1micron to about 500 microns, including 1 micron, 2 microns, 3 microns, 4micron, 5, microns, 6, microns, 7 microns, 8 microns, 9 microns, 10microns, 15 microns, 20 microns, 25 microns, 30 microns, 35 microns, 40microns, 45 microns, 50 microns, 60 microns, 70 microns, 80 microns, 90microns, 100 microns, 150 microns, 200 microns, 250 microns, 300microns, 350 microns, 400 microns, 450 microns, 500 microns, and allvalues in between. CNTs can also be less than about 1 micron in length,including about 100 nanometers, for example. CNTs can also be greaterthan 500 microns, including for example, 510 microns, 520 microns, 550microns, 600 microns, 700 microns and all values in between. For radarabsorbing applications, the CNTs can vary in length from between about 5to about 250 microns.

Radar absorbing composite materials of the invention can incorporateCNTs have a length from about 100 nanometers to about 10 microns. SuchCNT lengths can be useful in application to increase shear strength.CNTs can also have a length from about 5 to about 70 microns. Such CNTlengths can be useful in applications for increased tensile strength ifthe CNTs are aligned in the fiber direction. CNTs can also have a lengthfrom about 10 microns to about 100 microns. Such CNT lengths can beuseful to increase electrical/thermal properties as well as mechanicalproperties. The process used in the invention can also provide CNTshaving a length from about 100 microns to about 500 microns, which canalso be beneficial to increase electrical and thermal properties. Acomposite composed of a plurality of materials with the above CNTlengths is beneficial to radar absorption.

Thus, the CNT-infused fiber material is multifunctional and can enhancemany other properties of the overall radar absorbing composite. In someembodiments, composites that include spoolable lengths of CNT-infusedfiber materials can have various uniform regions with different lengthsof CNTs. For example, it can be desirable to have a first portion ofCNT-infused carbon fiber material with uniformly shorter CNT lengths toenhance shear strength properties, and a second portion of the samespoolable material with a uniform longer CNT length to enhance radarabsorption properties. For example, mechanical enhancement can beachieved by providing at least a portion of the radar absorbingcomposite material with shorter CNTs, as described above, in aCNT-infused fiber material. The composite can take the form of a skinhaving longer CNTs at the surface of the radar absorbing composite forradar absorption and shorter CNTs disposed below the surface formechanical strengthening. The control of CNT length is readily achievedthrough modulation of carbon feedstock and inert gas flow rates coupledwith varying linespeeds and growth temperature. This can vary the CNTlength in different sections of the same spoolable length of fibermaterial or different spools can be employed and the different spoolsincorporated in the appropriate portion of the composite structure.

Radar absorbing composite materials of the present invention include amatrix material to form the composite with the CNT-infused fibermaterial. Such matrix materials can include, for example, an epoxy, apolyester, a vinylester, a polyetherimide, a polyetherketoneketone, apolyphthalamide, a polyetherketone, a polytheretherketone, a polyimide,a phenol-formaldehyde, and a bismaleimide. Matrix materials useful inthe present invention can include any of the known matrix materials (seeMel M. Schwartz, Composite Materials Handbook (2 d ed. 1992)). Matrixmaterials more generally can include resins (polymers), boththermosetting and thermoplastic, metals, ceramics, and cements.

Thermosetting resins useful as matrix materials include phthalic/maelictype polyesters, vinyl esters, epoxies, phenolics, cyanates,bismaleimides, and nadic end-capped polyimides (e.g., PMR-15).Thermoplastic resins include polysulfones, polyamides, polycarbonates,polyphenylene oxides, polysulfides, polyether ether ketones, polyethersulfones, polyamide-imides, polyetherimides, polyimides, polyarylates,and liquid crystalline polyester.

Metals useful as matrix materials include alloys of aluminum such asaluminum 6061, 2024, and 713 aluminum braze. Ceramics useful as matrixmaterials include carbon ceramics, such as lithium aluminosilicate,oxides such as alumina and mullite, nitrides such as silicon nitride,and carbides such as silicon carbide. Cements useful as matrix materialsinclude carbide-base cermets (tungsten carbide, chromium carbide, andtitanium carbide), refractory cements (tungsten-thoria andbarium-carbonate-nickel), chromium-alumina, nickel-magnesiairon-zirconium carbide. Any of the above-described matrix materials canbe used alone or in combination. Ceramic or metal matrix composites canbe used in high temperature applications such as thrust vectoringsurfaces which can benefit from signature control materials.

In some embodiments, the radar absorbing composite can further include aplurality of transition metal nanoparticles. These transition metalnanoparticles can be present as latent catalyst from the CNT growthprocedure in some embodiments. Without being bound by theory, transitionmetal NPs, which serve as a CNT-forming catalyst, can catalyze CNTgrowth by forming a CNT growth seed structure. The CNT-forming catalystcan remain at the base of the fiber material, locked by a barriercoating (vide infra), if present, and infused to the surface of thefiber material. In such a case, the seed structure initially formed bythe transition metal nanoparticle catalyst is sufficient for continuednon-catalyzed seeded CNT growth without allowing the catalyst to movealong the leading edge of CNT growth, as often observed in the art. Insuch a case, the NP serves as a point of attachment for the CNT to thefiber material. The presence of a barrier coating can also lead tofurther indirect bonding motifs for CNT infusion. For example, the CNTforming catalyst can be locked into a barrier coating, as describedabove, but not in surface contact with fiber material. In such a case astacked structure with the barrier coating disposed between the CNTforming catalyst and fiber material results. In either case, the CNTsformed are infused to the fiber material. In some embodiments, somebarrier coatings will allow the CNT growth catalyst to follow theleading edge of the growing nanotube. In such cases, this can result indirect bonding of the CNTs to the fiber material or, optionally, to abarrier coating. Regardless of the nature of the actual bonding motifformed between the carbon nanotubes and the fiber material, the infusedCNT is robust and allows the CNT-infused fiber material to exhibitcarbon nanotube properties and/or characteristics.

In the absence of a barrier coating the latent CNT growth particles canappear at the base of the carbon nanotube, at the tip of the nanotube,anywhere in between, and mixtures thereof. Again, the infusion of theCNT to the fiber material can be either direct or indirect via theintervening transition metal nanoparticle. In some embodiments, thelatent CNT growth catalyst includes iron nanoparticles. These may be ofvarying oxidation state including, for example, zero-valent iron, iron(II), iron (III), and mixtures thereof The presence of latent iron basednanoparticles from CNT growth can further aid the radar absorptionproperty of the overall composite material.

In some embodiments, the CNT-infused fiber can be passed through aniron, ferrite, or iron-based nanoparticle solution post growth. CNTs canabsorb large quantities of iron nanoparticles which can further aid insignature control. Thus, this additional processing step providessupplemental iron nanoparticles for improved radar absorption bymechanisms analogous to iron ball paint.

Radar absorbing composite materials of the invention can absorb radaracross the entire spectrum of radar frequency bands. In someembodiments, the composite materials can absorb high frequency radar.High frequency (HF) radar bands have frequencies in a range from betweenabout 3 to about 30 MHz (10-100 m). This radar band is useful in coastalradar and over-the-horizon radar (OTH) radar applications. In someembodiments, the composite materials can absorb radar in the P-band.This includes radar frequencies less than about 300 MHz. In someembodiments, the composite materials can absorb radar in the very highfrequency band (VHF). VHF radar bands have frequencies in a range frombetween about 30 to about 330 MHz. The VHF band is useful inapplications that are very long range, including, ground penetratingapplications. In some embodiments, the composite materials can absorbradar in the ultra high frequency (UHF) band. The UHF band includesfrequencies in a range from between about 300 to about 1000 MHz.Applications of the UHF band include very long range applications, suchas ballistic missile early warning systems, ground penetrating andfoliage penetrating applications. In some embodiments, the compositematerials can absorb radar in the long (L) band. The L-band includesfrequencies in a range from between about 1 to about 2 GHz. The L-bandcan be useful in long range applications including, for example, airtraffic control and surveillance. In some embodiments, the compositematerials can absorb radar in the short (S)-band. The S-band includesfrequencies in a range from between about 2 to about 4 GHz. The S-bandcan be useful in applications such as terminal air traffic control,long-range weather, and marine radar. In some embodiments, the compositematerial can absorb radar in the C-band which has frequencies in a rangefrom between about 4 to about 8 GHz. The C-band has been used insatellite transponders and in weather applications. In some embodiments,the composite material can absorb radar in the X-band which hasfrequencies that range from between about 8 to about 12 GHz 2. TheX-band is useful in applications such as missile guidance, marine radar,weather, medium-resolution mapping and ground surveillance. In someembodiments, the composite material can absorb radar in the K-band whichincludes frequencies between about 12 to about 18 GHz. The K-band can beused for detecting clouds by meteorologists, and used by police fordetecting speeding motorists employing K-band radar guns. In someembodiments, the composite materials absorbs radar in the K_(a)-bandwhich includes frequencies from between about 24 to about 40 GHz. TheK_(a)-band can be used in photo radar, such as those used to triggercameras at traffic signals.

In some embodiments, the composite material absorbs radar in themillimeter (mm) band which is broadly between about 40 to about 300 GHz.The mm-band includes the Q-band from between about 40 to about 60 GHzwhich is used in military communication, the V-band from between about50 to about 75 GHz, which is strongly absorbed by atmospheric oxygen,the E-band from between about 60 to about 90 GHz, the W-band frombetween about 75 to about 110 GHz, which is used as a visual sensor forexperimental autonomous vehicles, high-resolution meteorologicalobservation, and imaging, and the UWB-band from between about 1.6 toabout 10.5 GHz, which is used for through-the-wall radar and imagingsystems.

In some embodiments, the composite material includes CNTs present in arange between about 1% by weight to about 5% by weight of the radarabsorbing composite material. In some embodiments, CNT loading can bebetween about 1% to about 20% by weight of the radar absorbing compositematerial. In some embodiments, CNT loading in the radar absorbingcomposite can be 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%,14%, 15%, 16%, 17%, 18%, 19%, and 20% by weight of the radar absorbingcomposite, including any fraction in between these values. CNT loadingin the radar absorbing composite can also be less than 1% including forexample between about 0.001% to about 1%. CNT loading the radarabsorbing composite can also be greater than 20% including, for example,25%, 30%, 40%, and so on up to about 60% and all values in between.

In some embodiments, a radar absorbing composite includes a carbonnanotube (CNT)-infused carbon fiber material. The CNT-infused carbonfiber material can include a carbon fiber material of spoolabledimensions, a barrier coating conformally disposed about the carbonfiber material, and carbon nanotubes (CNTs) infused to the carbon fibermaterial. The infusion of CNTs to the carbon fiber material can includea bonding motif of direct bonding of individual CNTs to the carbon fibermaterial or indirect bonding via a transition metal NP, barrier coating,or both.

CNT-infused carbon fiber materials of the invention can include abarrier coating. Barrier coatings can include for example analkoxysilane, methylsiloxane, an alumoxane, alumina nanoparticles, spinon glass and glass nanoparticles. The CNT-forming catalyst can be addedto the uncured barrier coating material and then applied to the carbonfiber material together. In other embodiments the barrier coatingmaterial can be added to the carbon fiber material prior to depositionof the CNT-forming catalyst. The barrier coating material can be of athickness sufficiently thin to allow exposure of the CNT-formingcatalyst to the carbon feedstock for subsequent CVD growth. In someembodiments, the thickness is less than or about equal to the effectivediameter of the CNT-forming catalyst. In some embodiments, the thicknessof the barrier coating is in a range from between about 10 nm to about100 nm. The barrier coating can also be less than 10 nm, including 1 nm,2 nm, 3nm, 4 nm, 5 nm, 6 nm, 7 nm, 8 nm, 9 nm, 10 nm, and any value inbetween.

Without being bound by theory, the barrier coating can serve as anintermediate layer between the carbon fiber material and the CNTs andcan provide mechanical infusion of the CNTs to the carbon fibermaterial. Such mechanical infusion still provides a robust system inwhich the carbon fiber material serves as a platform for organizing theCNTs while still imparting properties of the CNTs to the carbon fibermaterial. Moreover, the benefit of including a barrier coating is theimmediate protection it provides the carbon fiber material from chemicaldamage due to exposure to moisture and/or any thermal damage due toheating of the carbon fiber material at the temperatures used to promoteCNT growth.

When growing CNTs on carbon fiber materials, the elevated temperaturesand/or any residual oxygen and/or moisture that can be present in thereaction chamber can damage the carbon fiber material. Moreover, thecarbon fiber material itself can be damaged by reaction with theCNT-forming catalyst itself That is the carbon fiber material can behaveas a carbon feedstock to the catalyst at the reaction temperaturesemployed for CNT synthesis. Such excess carbon can disturb thecontrolled introduction of the carbon feedstock gas and can even serveto poison the catalyst by overloading it with carbon. The barriercoating employed in the invention is designed to facilitate CNTsynthesis on carbon fiber materials. Without being bound by theory, thecoating can provide a thermal barrier to heat degradation and/or can bea physical barrier preventing exposure of the carbon fiber material tothe environment at the elevated temperatures. Alternatively oradditionally, it can minimize the surface area contact between theCNT-forming catalyst and the carbon fiber material and/or it canmitigate the exposure of the carbon fiber material to the CNT-formingcatalyst at CNT growth temperatures.

There are three types of carbon fiber which are categorized based on theprecursors used to generate the fibers, any of which can be used in theinvention: Rayon, Polyacrylonitrile (PAN) and Pitch. Carbon fiber fromrayon precursors, which are cellulosic materials, has relatively lowcarbon content at about 20% and the fibers tend to have low strength andstiffness. Polyacrylonitrile (PAN) precursors provide a carbon fiberwith a carbon content of about 55%. Carbon fiber based on a PANprecursor generally has a higher tensile strength than carbon fiberbased on other carbon fiber precursors due to a minimum of surfacedefects.

Pitch precursors based on petroleum asphalt, coal tar, and polyvinylchloride can also be used to produce carbon fiber. Although pitches arerelatively low in cost and high in carbon yield, there can be issues ofnon-uniformity in a given batch.

In some embodiments, the CNT-infused fiber material includes a glassfiber material. CNT-infused glass fiber materials need not incorporate abarrier coating as described above, although one can be optionallyemployed. The glass-type used in the glass fiber material can be anytype, including for example, E-glass, A-glass, E-CR-glass, C-glass,D-glass, R-glass, and S-glass. E-glass includes alumino-borosilicateglass with less than 1% by weight alkali oxides and is mainly used forglass-reinforced plastics. A-glass includes alkali-lime glass withlittle or no boron oxide. E-CR-glass includes alumino-lime silicate withless than 1% by weight alkali oxides and has high acid resistance.C-glass includes alkali-lime glass with high boron oxide content and isused, for example, for glass staple fibers. D-glass includesborosilicate glass and possesses a high dielectric constant. R-glassincludes alumino silicate glass without MgO and CaO and possesses highmechanical strength. S-glass includes alumino silicate glass without CaObut with high MgO content and possesses high tensile strength. One ormore of these glass types can be processed into the glass fibermaterials described above. In particular embodiments, the glass isE-glass. In other embodiments, the glass is S-glass.

In some embodiments, if the CNT-infused fiber material includes aceramic fiber material. Like glass, the use of a barrier coating isoptional when using ceramic fiber materials. The ceramic-type used in aceramic fiber material can be any type, including for example, oxidessuch as alumina and zirconia, carbides, such as boron carbide, siliconcarbide, and tungsten carbide, and nitrides, such as boron nitride andsilicon nitride. Other ceramic fiber materials include, for example,borides and silicides. Ceramic fibers can also include basalt fibermaterials. Ceramic fiber materials may occur as composite materials withother fiber types. It is common to find fabric-like ceramic fibermaterials that also incorporate glass fiber, for example.

The CNT-infused fiber materials can include a fiber material based on afilament, a yarn, a tow, a tape, a fiber-braid, a woven fabric, anon-woven fiber mat, a fiber ply, and other 3D woven structures.Filaments include high aspect ratio fibers having diameters ranging insize from between about 1 micron to about 100 microns. Fiber tows aregenerally compactly associated bundles of filaments and are usuallytwisted together to give yarns.

Yarns include closely associated bundles of twisted filaments. Eachfilament diameter in a yarn is relatively uniform. Yarns have varyingweights described by their ‘tex,’ expressed as weight in grams of 1000linear meters, or denier, expressed as weight in pounds of 10,000 yards,with a typical tex range usually being between about 200 tex to about2000 tex, although this value will depend on the exact fiber materialbeing used.

Tows include loosely associated bundles of untwisted filaments. As inyarns, filament diameter in a tow is generally uniform. Tows also havevarying weights and the tex range is usually between 200 tex and 2000tex. They are frequently characterized by the number of thousands offilaments in the tow, for example 12K tow, 24K tow, 48K tow, and thelike. Again these values vary depending on the type of fiber materialbeing employed.

Tapes are materials that can be assembled as weaves or can representnon-woven flattened tows. Tapes can vary in width and are generallytwo-sided structures similar to ribbon. Processes of the presentinvention are compatible with CNT infusion on one or both sides of atape. CNT-infused tapes can resemble a “carpet” or “forest” on a flatsubstrate surface. Again, processes of the invention can be performed ina continuous mode to functionalize spools of tape.

Fiber-braids represent rope-like structures of densely packed carbonfibers. Such structures can be assembled from yarns, for example.Braided structures can include a hollow portion or a braided structurecan be assembled about another core material.

In some embodiments a number of primary fiber material structures can beorganized into fabric or sheet-like structures. These include, forexample, woven fabrics, non-woven fiber mat and fiber ply, in additionto the tapes described above. Such higher ordered structures can beassembled from parent tows, yarns, filaments or the like, with CNTsalready infused in the parent fiber. Alternatively such structures canserve as the substrate for the CNT infusion processes described herein.

FIG. 1-6 shows TEM and SEM images of CNTs prepared on carbon fibermaterials prepared by the processes described herein. The procedures forpreparing these materials are further detailed below and in ExamplesI-III. These Figures and procedures exemplify the process for carbonfiber materials, however, one skilled in the art will recognize thatother fiber materials can be employed, such as glass or ceramic, withoutsignificantly departing from these processes. FIGS. 1 and 2 show TEMimages of multi-walled and double-walled carbon nanotubes, respectively,that were prepared on an AS4 carbon fiber in a continuous process. FIG.3 shows a scanning electron microscope (SEM) image of CNTs growing fromwithin the barrier coating after the CNT-forming nanoparticle catalystwas mechanically infused to a carbon fiber material surface. FIG. 4shows a SEM image demonstrating the consistency in length distributionof CNTs grown on a carbon fiber material to within 20% of a targetedlength of about 40 microns. FIG. 5 shows an SEM image demonstrating theeffect of a barrier coating on CNT growth. Dense, well aligned CNTs grewwhere barrier coating was applied and no CNTs grew where barrier coatingwas absent. FIG. 6 shows a low magnification SEM of CNTs on carbon fiberdemonstrating the uniformity of CNT density across the fibers withinabout 10%.

Referring now to FIG. 7, there is illustrated schematically across-sectional view of a composite material 100, according to someembodiments of the invention. Composite material 100 is suitable forfabricating structures, for example aircraft components, havingdesirable radar absorbing characteristics. Composite material 100includes a plurality of fibers or filaments 110, such as in a tow orroving, that might be present in a matrix 140. Fibers 110 are infusedwith carbon nanotubes 120. In an exemplary embodiment, fibers 110 may beglass (e.g., E-glass, S-glass, D-glass) fibers. In another embodiment,fibers 110 may be carbon (graphite) fibers. Other fibers such aspolyamide (Aromatic polyamide, Aramid) (e.g., Kevlar 29 and Kevlar 49),metallic fiber (e.g., steel, aluminum, molybdenum, tantalum, titanium,copper, and tungsten), tungsten monocarbide, ceramic fiber,metallic-ceramic fiber (e.g., aluminum silica), cellulosic fiber,polyester, quartz, and silicon carbide may also be used. CNT synthesisprocesses described herein with regard to carbon fibers can be used forCNT synthesis on any fiber type. In some embodiments, the metallicfibers can be coated with an appropriate barrier coating before applyingthe catalyst particles thereto, to prevent undesirable chemical reactionbetween the catalyst particles and the metallic fibers such as alloying.Thus, when employing metallic fiber materials, the process can parallelthat used for carbon fiber materials. Similarly, the thermal sensitivearamid fibers can also employ a barrier coating to protect the fibermaterial from the typical temperature employed during CNT growth.

In an exemplary embodiment, carbon nanotubes 120 may be grown generallyperpendicularly from the outer surface of fiber 110, thereby providing aradial coverage to each individual fiber 110. Carbon nanotubes 120 maybe grown in situ on fibers 110. For example, a glass fiber 110 may befed through a growth chamber maintained at a given temperature of about500° to 750° C. Carbon containing feed gas can then be introduced intothe growth chamber, wherein carbon radicals dissociate and initiateformation of carbon nanotubes on the glass fiber, in presence of thecatalyst nanoparticles.

FIG. 13 shows an exemplary RAM panel 1300, in accordance with thepresent invention. RAM panel 1300 represents a cross section ofCNT-infused fiber-based radar absorbing material. First layer 1310 is incontact with the incoming EM waves (radar) and has a dielectric constantsimilar to air. First layer 1310 can be a glass fiber composite materialin some embodiments. The thickness of first layer 1310 is sized in orderto take advantage of the quarter wavelength rule given the refractiveindex of the second layer 1320 which is a heavily loaded CNT-infusedfiber material. CNT quantities of 1%-60% can be targeted in thisstructure depending on the specific EM frequency of interest and theresulting refractive index desired. The high CNT content provides a highdielectric constant, a more conductive material, which will reflectincoming EM waves. These reflected waves, with properly sized firstlayer 1310 will internally reflect and dissipate as heat.

FIG. 14 shows another exemplary RAM panel 1400. RAM panel 1400represents a cross section of a multi-layered CNT-infused fiber basedradar absorbing material. Such embodiments use varying amounts of CNTsin each successive layer to induce internal reflection and EM wave(radar) dissipation of multiple frequencies using one panel structure.RAM panel 1400 has a first layer 1410, which is exposed to the incidentEM wave, intermediate layer 1420, and a second layer 1430. It will beapparent to one skilled in the art that intermediate layer 1420 canexist as any number of multiple intermediate layers with increasing CNTcontent moving from the first layer to the second layer. Continuing inthis fashion, first layer 1410 has a dielectric constant similar to airto allow for wave transmittance. As a result, this material has low %CNTs between 0-1% CNTs by weight in the composite structure. First layer1410 is sized such that its thickness takes advantage of the quarterwavelength theory for total internal reflection which is dependent onthe incoming frequency of the EM wave and the index of refraction ofintermediate layer 1420. Intermediate layer 1420 consists of CNT weight% 0.1-5% in composite and are similarly sized depending on refractivecharacteristics of second layer 1430 or any successive intermediatelayers used as well as the frequencies of interest. Second layer 1430consists of the highest weight % CNTs, between 1-60% and is sizedsimilarly to the first layer and intermediate layers. Second layer 1430is typically maximally loaded with CNTs to provide the most reflectivesurface.

In some embodiments, the CNTs are infused on a fiber material in acontinuous density gradient along the fiber. Such embodiments areanalogous to that shown in FIG. 14. A continuous gradient RAM structureuses a continuously variable amount of CNTs on a fiber material suchthat the resulting wound structure contains tailored and constantlyvarying amounts of CNTs at specific depths targeted at absorbing andinternally reflecting EM waves more evenly across the entire spectrum ofinterest. A continuously variable structure can provide the ability tocreate a highly effective radar absorbing material across a broadspectrum instead of a RAM that targets specific peaks. This is madepossible by layered arrangements as described in FIG. 14.

In one configuration, to create composite material 100, CNT-infusedfiber 110 is delivered to a resin bath. In another configuration, afabric may be woven from CNT-infused fibers 110 and the fabricsubsequently delivered to a resin bath. The resin bath can contain anyresin for the production of composite material 100 comprisingCNT-infused fibers 110 and matrix 140. In one configuration, matrix 140may take the form of an epoxy resin matrix. In another configuration,matrix 140 may be one of general purpose polyester (such asorthophthalic polyesters), improved polyester (such as isophthalicpolyesters), phenolic resin, bismaleimide (BMI) resin, polyurethane, andvinyl ester. Matrix 140 can also take the form of a non-resin matrix(for example, a ceramic matrix) useful for applications requiringperformance at higher operational temperatures, such as aerospace and/ormilitary applications. It will be understood that matrix 140 can alsotake the form of a metal matrix.

Known composite manufacturing methods such as vacuum assisted resininfusion method and resin extrusion method for impregnating CNT-infusedfibers 110, or a fabric woven therefrom, with a resin matrix may beapplied. For example, CNT-infused fibers 110, or a fabric thereof, maybe laid in a mold and resin may be infused therein. In anotherconfiguration, CNT-infused fibers 110, or a fabric thereof, may be laidin a mold, which is then evacuated to pull the resin therethrough. Inanother configuration, CNT-infused fibers 110 may be woven in a “0/90”orientation by winding. This may be accomplished, for example, bywinding a first layer or panel of CNT-infused fibers 100 in a firstdirection, such as the vertical direction, and then winding a secondlayer or panel of CNT-infused fibers 110 in a second direction, such asthe horizontal direction, which is about 90° to the first direction.Such a “0/90” orientation can impart additional structural strength tocomposite material 100.

Fibers 110 infused with carbon nanotubes 120 can be incorporated in athermoset plastic matrix (e.g., an epoxy resin matrix) 140 to createcomposite material 100. The methods for incorporating fibers in a matrixare well known in the art. In one configuration, CNT-infused fibers 110can be incorporated in matrix 140 using a high pressure curing method.CNT loading of a composite signifies the weight percentage of carbonnanotubes in a given composite. Processes known in the art for producingCNT-based composites involve direct mixing of loose (i.e. not bound tospoolable length fibers) carbon nanotubes into the resin/matrix of thenascent composite. The composites resulting from such processes are canbe limited to a maximum of about five (5) weight percent of carbonnanotubes in the finished composite material due to factors such asprohibitive viscosity increases. Composite material 100, on the otherhand, may have a CNT loading in excess of 25 weight %, as describedherein above. Using CNT-infused fibers 110, composite materials havingCNT loading as high as 60 weight percent have been demonstrated. Theradar absorbing characteristic of a material can depend, in part, on itselectrical conductivity. Overall electrical conductivity of composite100 is, in part, a function of the CNT loading of composite 100.

The above-described composite material 100 with CNT-infused fibersincorporated therein is suitable for fabricating components withelectromagnetic or radar absorbing characteristics, for aircrafts, andsubmarines, for example. It has been demonstrated that compositematerial 100 effectively absorbs electromagnetic radiation in the radarspectrum, including infrared (about 700 nm to about 15 centimeters),visible (about 400 nm to about 700 nm) and ultraviolet (about 10 nm toabout 400 nm) radiation.

Composite structures which are desirable, for example, for their weightand strength characteristics, are sometimes not suitable for fabricatingaircraft components because of their relatively poor signature control,or radar absorbing characteristics. For example, carbon fiber compositesare generally reflective of radar waves and therefore have relativelypoor signature control. Glass fiber composites, on the other hand, aregenerally transparent to radar waves. However, they are generallydielectric in nature and have poor electrical and thermalconductivities. Incorporation of CNTs in carbon fiber composites andglass fiber composites effectively enhances radar wave absorptivity ofthe resulting composite materials. In the case of glass fibercomposites, incorporated CNTs also improve thermal and electricalconductivities of the resulting composite materials. Composite 100 withCNT-infused fibers 110, thus, enhances the signature controlcharacteristics, while retaining the desirable characteristics such aslow weight to strength ratio associated with composite materials. Theeffectiveness of a composite material as a radar absorbent can beadjusted by tailoring the weight percentage of carbon nanotubes in thecomposite material. Without being bound by theory, the incorporated CNTscan absorb radar waves in a fashion similar to iron nanoparticles usedin iron ball paints.

Referring now to FIG. 8, a cross-sectional view of a CNT-infused fibermaterial 200 is schematically illustrated. Fiber material 200 canoptionally include a matrix. Regardless of the existence of a matrixmaterial, CNT-infused fiber material 200 can be applied to a surface ofa previously fabricated composite material to significantly enhanceradar absorbing or signature control characteristics of the compositematerial. In some embodiments, the pre-fabricated composite material, onits own, can exhibit poor signature control. However, the CNT-infusedfiber material disposed on its surface can impart a sufficient degree ofradar absorbing capacity to provide good signature control. CNT-infusedfiber material 200 can be wound or woven about the pre-fabricatedcomposite material. In some embodiments, where a matrix material was notpreviously present with CNT-infused fiber material 200 prior todisposing it on the composite material, one can be added after it isdisposed thereon. Moreover, the matrix material added thusly can be ofthe same matrix as the pre-fabricated material, or of similarcharacteristics to promote strong bonding.

CNT-infused fiber material 200 includes a plurality of fibers in a fibermaterial 210, such as a tow or roving. Carbon nanotubes 120 are infusedto fiber material 210. Van der Waals forces between closely-situatedgroups of carbon nanotubes 120 can provide a significant increase in theinteraction between CNTs 120. In some embodiments, this can result inCNT “interdigitation” of carbon nanotubes 120, which can provide afilament-to-filament bond or adhesion. In an exemplary embodiment, theinterdigitation of carbon nanotubes 120 may be further induced byapplying pressure to fiber material 210 in order to consolidateCNT-infused fiber material 200. This filament-to-filament bond canenhance the formation of fiber tows, tapes, and weaves in the absence ofa resin matrix. This filament-to-filament bond can also increase shearand tensile strengths, relative to a filament-resin bond as might beemployed in conventional fiber tow composites. Composite fiber materialsformed from such CNT-infused fiber tows exhibit good signature controland/or radar absorbing characteristics along with increased interlaminarshear strength, tensile strength, and out-of-axis strength. In someembodiments, the CNTs need not be fully interdigitated to providebeneficial radar absorbing characteristics. For example, percolationpathways can be created by simple point contact between CNTs.

In one configuration, CNT-infused fiber materials 200 can be applied asa coating on a surface of a conventional composite material, such as aglass fiber composite panel or a carbon fiber composite panel, to impartgood signature control characteristics to such conventional compositematerial. In one configuration, CNT-infused fiber materials 200 may bewound around a composite structure to enhance radar absorbing orsignature control characteristics of the composite structure. A coatingof a matrix, such as a resin matrix, can be applied over one or morelayers of CNT-infused fiber materials 200, or a fabric woven therefrom,applied to a surface of the composite material to protect CNT-infusedcomposite fibers 200 from external environment. Multiple layeredCNT-infused fiber materials can be disposed to provide multiple CNTorientations, lengths, and densities to vary the radar absorptioncharacteristics to absorb radar in different frequency bands and toabsorb radar from sources that impinge the overall structure fromdifferent angles.

Referring now to FIG. 9, there is illustrated schematically a coatinglayer of fiber material 210 with infused CNTs disposed on a top surface355 of a composite material 350. Composite 350 may take the form of aconventional composite glass or glass-reinforced plastic, for example.In another configuration, composite 350 may take the form of a carbonfiber composite structure or a carbon fiber reinforced plasticstructure. Composite 350, on its own, is generally not suitable for usein applications requiring good radar absorbing or signature controlcharacteristics. However, by applying a coating or layer 230 of fibermaterial 210 having CNTs infused thereon, onto surface 355 of composite350, the combination (i.e., the combination of composite 350 andCNT-infused fibers) exhibits significantly enhanced radar absorbing orsignature control characteristics. In an exemplary embodiment, fibers210 may be a fiber tow infused with carbon nanotubes 220 with a matrix,such as, a resin matrix. In yet another exemplary embodiment, fibers 210may be woven to form a fabric, which may be applied to top surface 355of composite material 350.

In some embodiments, CNT-infused fiber materials 200 may be woven toform a fabric. In one configuration, a coating of fibers can have athickness ranging from about 20 nanometers (nm) for a single layer ofCNT-infused fibers to about 12.5 mm for multiple layers of CNT-infusedfibers. While the illustrated embodiment depicts a single layer offibers for the sake of simplicity, it will be understood that multiplelayers of fibers can be used to form a coating on composite 350.

An advantage of using CNT-infused fiber material 200 is that such acoating can be used in conjunction with conventional composite materialshaving poor radar absorbing or signature control characteristics for lowobservable applications while retaining advantages of the composite suchas weight to strength ratios and other desirable mechanical andstructural characteristics.

A layer or coating of CNT-infused fiber material 200 can be disposed ona surface of a composite structure such as the leading edges of wingstructures of an aircraft to enhance the radar absorbing or signaturecontrol characteristics of the composite structure. Such a use of alayer or coating of CNT-infused fiber material 200 applied to aconventional composite material facilitates using conventional compositematerials for fabrication, for example, a wing structure and othercomponents of an aircraft, and reducing the weight of the aircraftsignificantly while simultaneously having low observablecharacteristics.

Referring now to FIG. 10, there is illustrated a coating system 400,according to an exemplary embodiment. System 400 receives CNT-infusedfiber 110 from an upstream fiber source. In an exemplary embodiment,CNT-infused fibers can be directed to coating system 400 directly fromthe growth chamber where carbon nanotubes 120 are infused onto the fibermaterial. CNT-infused fiber 110 is immersed in a chemical solution 420contained in a bath 410 to further treat CNT-infused fiber 110.CNT-infused fiber 110 is guided by two guide rollers 440, 450. A bathroller 430 immerses CNT-infused fiber 110 into solution 420. In anexemplary embodiment, solution 420 is an iron-based nanoparticlesolution. In one configuration, solution 420 includes 1 part volume ironbased solute in 200 parts hexane solvent. Carbon nanotubes 120 onCNT-infused fiber 110 will absorb iron nanoparticles, thereby furtherenhancing radar absorbing or signature control characteristics ofCNT-infused fiber 110 and any composite fabricated therefrom. It will beunderstood that broad band fabrics fabricated from CNT-infused fibers110 may similarly be treated to incorporate iron based nanoparticles.

In some embodiments, the radar absorbing composite material can haveCNTs infused on the fiber material in a controlled manner. For example,the CNTs may be grown in a dense radial display about individual fiberelements of the fiber material. In other embodiments, the CNTs can beprocessed further post growth to align directly along the fiber axis. Inorder to produce a CNT-Infused composite coating, a traditional fiber onthe scale of 1-15 microns can be used as a surface for synthesis.Various surface modification techniques and additional coatings can beused to protect the fiber and/or improve fiber to catalyst interfaces.Catalyst coatings are applied via any number of spray, dip, and gasphase processes. Once a layer of catalyst has been deposited, catalystreduction and CNT growth occur simultaneously during a CVD based growthprocess in a atmospheric pressure growth system, in situ continuously.After one layer of CNTs is grown, additional techniques, beyond thosementioned in the patent application, must be employed to align theradial (as grown) CNTs in the direction of the fiber. Three broadlydescribed techniques as well as any combination of them can be used toachieve this alignment. These techniques are described as follows:

Electromechanical—Via the use of an electric or magnetic field alignedparallel to the fiber during the growth process, CNTs can be induced toalign by way of the force field applied.

Mechanical—A variety of mechanical techniques including extrusion,pultrusion, gas pressure aided dies, conventional dies, and mandrels canbe used to apply a shearing force in the direction of the fibers toinduce alignment.

Chemical—Chemicals including solvents, surfactants, and micro-emulsionscan be used to induce alignment via the sheathing effect in thedirection of the fibers observed as material is drawn out of thesechemicals.

Moreover, because the CNTs can have a defined orientation with respectto the fiber axis, the CNTs, in turn can have a controlled orientationwithin any overall composite structure made therefrom. This can beachieved in any of the winding and/or fabric processes described above,or by controlling orientation of the CNT-infused fiber material in the aresin matrix for curing or the like.

Thus, in some embodiments, the present invention provides a method ofmanufacturing these radar absorbing composite materials that includes 1)disposing a CNT-infused fiber material in a portion of a matrix materialwith a controlled orientation of the CNT-infused fiber material withinthe matrix material, and 2) curing the matrix material, wherein thecontrolled orientation of the CNT-infused fiber material controls therelative orientation of CNTs infused thereon.

Fibers can be incorporated into composites via wet winding, dry windingfollowed by vacuum assisted resin infusion, prepreg (resin in tacky formon fabric or fibers). In each case, the matrix is cured once the fibersand matrix are combined in a structure. Curing occurs under atmospheric,low pressure, or high pressure conditions.

In some embodiments, the present invention provides a panel thatincludes the radar absorbing composite materials of the invention. Thepanel can be made as a structural component of a transport vessel orprojectile for use in stealth applications, in some embodiments. Atransport vessel can include such a panel having the CNTs infused fibermaterial having CNTs in a controlled orientation within the compositematerial. The panel can be optionally equipped with a mechanism toadjust its angle with respect to an impinging angle of incidence of aradar transmitting source to maximize radar absorption. For example, theenergy of the absorbed radar signal can be used to convert to anelectrical signal which is integrated with a computer system to alterthe orientation of the panel to maximize radar absorption. This canprovide a means of optimizing against detection from multiple radarsources from previously unknown directions of impingement. In someembodiments, the transport vessel can take the form of a boat, a plane,and a ground vehicle, for example.

In some embodiments, the radar absorbing material can also be used toabsorb radar in detector applications, where a reflected radar signalrequires efficient capture. Thus, in addition to stealth application ofnot being detected. Radar absorbing composite materials of the presentinvention can be integrated into detection systems to more efficientlyreceive back reflected radar signals.

In some embodiments, radar absorbing materials of the invention can beincorporated in a projectile system. This can be achieved using a paneltype system or coating as described above. In such applications the CNTsinfused on the fiber material have a controlled orientation within thecomposite material and the panel or coating. When utilizing a panel, thepanel can further be equipped with a mechanism to adjust its angle withrespect to an impinging angle of incidence of a radar transmittingsource to maximize radar absorption. In this way, a projectile can bemade that can evade radar detection.

As described briefly above, the present invention relies on a continuousCNT infusion process to generate CNT-infused fiber materials. Theprocess includes (a) disposing a carbon nanotube-forming catalyst on asurface of a fiber material of spoolable dimensions; and (b)synthesizing carbon nanotubes directly on the carbon fiber material,thereby forming a carbon nanotube-infused fiber material. Additionalsteps can be employed depending on the type of fiber material beingused. For example, when using carbon fiber materials, a step thatincorporates a barrier coating can be added to the process.

For a 9 foot long system, the linespeed of the process can range frombetween about 1.5 ft/min to about 108 ft/min. The linespeeds achieved bythe process described herein allow the formation of commerciallyrelevant quantities of CNT-infused fiber materials with short productiontimes. For example, at 36 ft/min linespeed, the quantities ofCNT-infused fibers (over 5% infused CNTs on fiber by weight) can exceedover 100 pound or more of material produced per day in a system that isdesigned to simultaneously process 5 separate tows (20 lb/tow). Systemscan be made to produce more tows at once or at faster speeds byrepeating growth zones. Moreover, some steps in the fabrication of CNTs,as known in the art, have prohibitively slow rates preventing acontinuous mode of operation. For example, in a typical process known inthe art, a CNT-forming catalyst reduction step can take 1-12 hours toperform. CNT growth itself can also be time consuming, for examplerequiring tens of minutes for CNT growth, precluding the rapidlinespeeds realized in the present invention. The process describedherein overcomes such rate limiting steps.

The CNT-infused carbon fiber material-forming processes of the inventioncan avoid CNT entanglement that occurs when trying to apply suspensionsof pre-formed carbon nanotubes to fiber materials. That is, becausepre-formed CNTs are not fused to the carbon fiber material, the CNTstend to bundle and entangle. The result is a poorly uniform distributionof CNTs that weakly adhere to the carbon fiber material. However,processes of the present invention can provide, if desired, a highlyuniform entangled CNT mat on the surface of the carbon fiber material byreducing the growth density. The CNTs grown at low density are infusedin the carbon fiber material first. In such embodiments, the fibers donot grow dense enough to induce vertical alignment, the result isentangled mats on the carbon fiber material surfaces. By contrast,manual application of pre-formed CNTs does not insure uniformdistribution and density of a CNT mat on the carbon fiber material.

FIG. 11 depicts a flow diagram of process 700 for producing CNT-infusedcarbon fiber material in accordance with an illustrative embodiment ofthe present invention. One skilled in the art will recognize that slightvariations in this process exemplifying CNT infusion on a carbon fibermaterial can be altered to provide other CNT-infused fiber materialssuch as glass or ceramic fibers, for example. Some such alterations inthe conditions can include, for example, removing the step of applying abarrier coating, which is optional for glass and ceramics.

Process 700 includes at least the operations of:

701: Functionalizing the carbon fiber material.

702: Applying a barrier coating and a CNT-forming catalyst to thefunctionalized carbon fiber material.

704: Heating the carbon fiber material to a temperature that issufficient for carbon nanotube synthesis.

706: Promoting CVD-mediated CNT growth on the catalyst-laden carbonfiber.

In step 701, the carbon fiber material is functionalized to promotesurface wetting of the fibers and to improve adhesion of the barriercoating.

To infuse carbon nanotubes into a carbon fiber material, for example,the carbon nanotubes are synthesized on the carbon fiber material whichis conformally coated with a barrier coating. In one embodiment, this isaccomplished by first conformally coating the carbon fiber material witha barrier coating and then disposing nanotube-forming catalyst on thebarrier coating, as per operation 702. In some embodiments, the barriercoating can be partially cured prior to catalyst deposition. This canprovide a surface that is receptive to receiving the catalyst andallowing it to embed in the barrier coating, including allowing surfacecontact between the CNT forming catalyst and the carbon fiber material.In such embodiments, the barrier coating can be fully cured afterembedding the catalyst. In some embodiments, the barrier coating isconformally coated over the carbon fiber material simultaneously withdeposition of the CNT-form catalyst. Once the CNT-forming catalyst andbarrier coating are in place, the barrier coating can be fully cured.

In some embodiments, the barrier coating can be fully cured prior tocatalyst deposition. In such embodiments, a fully cured barrier-coatedcarbon fiber material can be treated with a plasma to prepare thesurface to accept the catalyst. For example, a plasma treated carbonfiber material having a cured barrier coating can provide a roughenedsurface in which the CNT-forming catalyst can be deposited. The plasmaprocess for “roughing” the surface of the barrier thus facilitatescatalyst deposition. The roughness is typically on the scale ofnanometers. In the plasma treatment process craters or depressions areformed that are nanometers deep and nanometers in diameter. Such surfacemodification can be achieved using a plasma of any one or more of avariety of different gases, including, without limitation, argon,helium, oxygen, nitrogen, and hydrogen. In some embodiments, plasmaroughing can also be performed directly in the carbon fiber materialitself. This can facilitate adhesion of the barrier coating to thecarbon fiber material.

As described further below and in conjunction with FIG. 11, the catalystis prepared as a liquid solution that contains CNT-forming catalyst thatcomprise transition metal nanoparticles. The diameters of thesynthesized nanotubes are related to the size of the metal particles asdescribed above. In some embodiments, commercial dispersions ofCNT-forming transition metal nanoparticle catalyst are available and areused without dilution, in other embodiments commercial dispersions ofcatalyst can be diluted. Whether to dilute such solutions can depend onthe desired density and length of CNT to be grown as described above.

With reference to the illustrative embodiment of FIG. 11, carbonnanotube synthesis is shown based on a chemical vapor deposition (CVD)process and occurs at elevated temperatures. The specific temperature isa function of catalyst choice, but will typically be in a range of about500 to 1000° C. Accordingly, operation 704 involves heating thebarrier-coated carbon fiber material to a temperature in theaforementioned range to support carbon nanotube synthesis.

In operation 706, CVD-promoted nanotube growth on the catalyst-ladencarbon fiber material is then performed. The CVD process can be promotedby, for example, a carbon-containing feedstock gas such as acetylene,ethylene, and/or ethanol. The CNT synthesis processes generally use aninert gas (nitrogen, argon, helium) as a primary carrier gas. The carbonfeedstock is provided in a range from between about 0% to about 15% ofthe total mixture. A substantially inert environment for CVD growth isprepared by removal of moisture and oxygen from the growth chamber.

In the CNT synthesis process, CNTs grow at the sites of a CNT-formingtransition metal nanoparticle catalyst. The presence of the strongplasma-creating electric field can be optionally employed to affectnanotube growth. That is, the growth tends to follow the direction ofthe electric field. By properly adjusting the geometry of the plasmaspray and electric field, vertically-aligned CNTs (i.e., perpendicularto the carbon fiber material) can be synthesized. Under certainconditions, even in the absence of a plasma, closely-spaced nanotubeswill maintain a vertical growth direction resulting in a dense array ofCNTs resembling a carpet or forest. The presence of the barrier coatingcan also influence the directionality of CNT growth.

The operation of disposing a catalyst on a carbon fiber material can beaccomplished by spraying or dip coating a solution or by gas phasedeposition via, for example, a plasma process. The choice of techniquescan be coordinated with the mode with which the barrier coating isapplied. Thus, in some embodiments, after forming a solution of acatalyst in a solvent, catalyst can be applied by spraying or dipcoating the barrier coated carbon fiber material with the solution, orcombinations of spraying and dip coating. Either technique, used aloneor in combination, can be employed once, twice, thrice, four times, upto any number of times to provide a carbon fiber material that issufficiently uniformly coated with CNT-forming catalyst. When dipcoating is employed, for example, a carbon fiber material can be placedin a first dip bath for a first residence time in the first dip bath.When employing a second dip bath, the carbon fiber material can beplaced in the second dip bath for a second residence time. For example,carbon fiber materials can be subjected to a solution of CNT-formingcatalyst for between about 3 seconds to about 90 seconds depending onthe dip configuration and linespeed. Employing spraying or dip coatingprocesses, a carbon fiber material with a surface density of catalyst ofless than about 5% surface coverage to as high as about 80% coverage, inwhich the CNT-forming catalyst nanoparticles are nearly monolayer. Insome embodiments, the process of coating the CNT-forming catalyst on thecarbon fiber material should produce no more than a monolayer. Forexample, CNT growth on a stack of CNT-forming catalyst can erode thedegree of infusion of the CNT to the carbon fiber material. In otherembodiments, the transition metal catalyst can be deposited on thecarbon fiber material using evaporation techniques, electrolyticdeposition techniques, and other processes known to those skilled in theart, such as addition of the transition metal catalyst to a plasmafeedstock gas as a metal organic, metal salt or other compositionpromoting gas phase transport.

Because processes of the invention are designed to be continuous, aspoolable carbon fiber material can be dip-coated in a series of bathswhere dip coating baths are spatially separated. In a continuous processin which nascent carbon fibers are being generated de novo, dip bath orspraying of CNT-forming catalyst can be the first step after applyingand curing or partially curing a barrier coating to the carbon fibermaterial. Application of the barrier coating and a CNT-forming catalystcan be performed in lieu of application of a sizing, for newly formedcarbon fiber materials. In other embodiments, the CNT-forming catalystcan be applied to newly formed carbon fibers in the presence of othersizing agents after barrier coating. Such simultaneous application ofCNT-forming catalyst and other sizing agents can still provide theCNT-forming catalyst in surface contact with the barrier coating of thecarbon fiber material to insure CNT infusion.

The catalyst solution employed can be a transition metal nanoparticlewhich can be any d-block transition metal as described above. Inaddition, the nanoparticles can include alloys and non-alloy mixtures ofd-block metals in elemental form or in salt form, and mixtures thereof.Such salt forms include, without limitation, oxides, carbides, andnitrides. Non-limiting exemplary transition metal NPs include Ni, Fe,Co, Mo, Cu, Pt, Au, and Ag and salts thereof and mixtures thereof. Insome embodiments, such CNT-forming catalysts are disposed on the carbonfiber by applying or infusing a CNT-forming catalyst directly to thecarbon fiber material simultaneously with barrier coating deposition.Many of these transition metal catalysts are readily commerciallyavailable from a variety of suppliers, including, for example, FerrotecCorporation (Bedford, N.H.).

Catalyst solutions used for applying the CNT-forming catalyst to thecarbon fiber material can be in any common solvent that allows theCNT-forming catalyst to be uniformly dispersed throughout. Such solventscan include, without limitation, water, acetone, hexane, isopropylalcohol, toluene, ethanol, methanol, tetrahydrofuran (THF), cyclohexaneor any other solvent with controlled polarity to create an appropriatedispersion of the CNT-forming catalyst nanoparticles. Concentrations ofCNT-forming catalyst can be in a range from about 1:1 to 1:10000catalyst to solvent. Such concentrations can be used when the barrier.coating and CNT-forming catalyst is applied simultaneously as well.

In some embodiments heating of the carbon fiber material can be at atemperature that is between about 500° C. and 1000° C. to synthesizecarbon nanotubes after deposition of the CNT-forming catalyst. Heatingat these temperatures can be performed prior to or substantiallysimultaneously with introduction of a carbon feedstock for CNT growth.

In some embodiments, the present invention provides a process thatincludes removing sizing agents from a carbon fiber material, applying abarrier coating conformally over the carbon fiber material, applying aCNT-forming catalyst to the carbon fiber material, heating the carbonfiber material to at least 500° C., and synthesizing carbon nanotubes onthe carbon fiber material. In some embodiments, operations of theCNT-infusion process include removing sizing from a carbon fibermaterial, applying a barrier coating to the carbon fiber material,applying a CNT-forming catalyst to the carbon fiber, heating the fiberto CNT-synthesis temperature and CVD-promoted CNT growth thecatalyst-laden carbon fiber material. Thus, where commercial carbonfiber materials are employed, processes for constructing CNT-infusedcarbon fibers can include a discrete step of removing sizing from thecarbon fiber material before disposing barrier coating and the catalyston the carbon fiber material.

The step of synthesizing carbon nanotubes can include numeroustechniques for forming carbon nanotubes, including those disclosed inco-pending U.S. Patent Application No. US 2004/0245088 which isincorporated herein by reference. The CNTs grown on fibers of thepresent invention can be accomplished by techniques known in the artincluding, without limitation, micro-cavity, thermal or plasma-enhancedCVD techniques, laser ablation, arc discharge, and high pressure carbonmonoxide (HiPCO). During CVD, in particular, a barrier coated carbonfiber material with CNT-forming catalyst disposed thereon, can be useddirectly. In some embodiments, any conventional sizing agents can beremoved prior CNT synthesis. In some embodiments, acetylene gas isionized to create a jet of cold carbon plasma for CNT synthesis. Theplasma is directed toward the catalyst-bearing carbon fiber material.Thus, in some embodiments synthesizing CNTs on a carbon fiber materialincludes (a) forming a carbon plasma; and (b) directing the carbonplasma onto the catalyst disposed on the carbon fiber material. Thediameters of the CNTs that are grown are dictated by the size of theCNT-forming catalyst as described above. In some embodiments, the sizedfiber substrate is heated to between about 550 to about 800° C. tofacilitate CNT synthesis. To initiate the growth of CNTs, two gases arebled into the reactor: a process gas such as argon, helium, or nitrogen,and a carbon-containing gas, such as acetylene, ethylene, ethanol ormethane. CNTs grow at the sites of the CNT-forming catalyst.

In some embodiments, the CVD growth is plasma-enhanced. A plasma can begenerated by providing an electric field during the growth process. CNTsgrown under these conditions can follow the direction of the electricfield. Thus, by adjusting the geometry of the reactor vertically alignedcarbon nanotubes can be grown radially about a cylindrical fiber. Insome embodiments, a plasma is not required for radial growth about thefiber. For carbon fiber materials that have distinct sides such astapes, mats, fabrics, plies, and the like, catalyst can be disposed onone or both sides and correspondingly, CNTs can be grown on one or bothsides as well.

As described above, CNT-synthesis is performed at a rate sufficient toprovide a continuous process for functionalizing spoolable carbon fibermaterials. Numerous apparatus configurations faciliate such continuoussynthesis as exemplified below.

In some embodiments, CNT-infused carbon fiber materials can beconstructed in an “all plasma” process. An all plasma process can beingwith roughing the carbon fiber material with a plasma as described aboveto improve fiber surface wetting characteristics and provide a moreconformal barrier coating, as well as improve coating adhesion viamechanical interlocking and chemical adhesion through the use offunctionalization of the carbon fiber material by using specificreactive gas species, such as oxygen, nitrogen, hydrogen in argon orhelium based plasmas.

Barrier coated carbon fiber materials pass through numerous furtherplasma-mediated steps to form the final CNT-infused product. In someembodiments, the all plasma process can include a second surfacemodification after the barrier coating is cured. This is a plasmaprocess for “roughing” the surface of the barrier coating on the carbonfiber material to facilitate catalyst deposition. As described above,surface modification can be achieved using a plasma of any one or moreof a variety of different gases, including, without limitation, argon,helium, oxygen, ammonia, hydrogen, and nitrogen.

After surface modification, the barrier coated carbon fiber materialproceeds to catalyst application. This is a plasma process fordepositing the CNT-forming catalyst on the fibers. The CNT-formingcatalyst is typically a transition metal as described above. Thetransition metal catalyst can be added to a plasma feedstock gas as aprecursor in the form of a ferrofluid, a metal organic, metal salt orother composition for promoting gas phase transport. The catalyst can beapplied at room temperature in the ambient environment with neithervacuum nor an inert atmosphere being required. In some embodiments, thecarbon fiber material is cooled prior to catalyst application.

Continuing the all-plasma process, carbon nanotube synthesis occurs in aCNT-growth reactor. This can be achieved through the use ofplasma-enhanced chemical vapor deposition, wherein carbon plasma issprayed onto the catalyst-laden fibers. Since carbon nanotube growthoccurs at elevated temperatures (typically in a range of about 500 to1000° C. depending on the catalyst), the catalyst-laden fibers can beheated prior to exposing to the carbon plasma. For the infusion process,the carbon fiber material can be optionally heated until it softens.After heating, the carbon fiber material is ready to receive the carbonplasma. The carbon plasma is generated, for example, by passing a carboncontaining gas such as acetylene, ethylene, ethanol, and the like,through an electric field that is capable of ionizing the gas. This coldcarbon plasma is directed, via spray nozzles, to the carbon fibermaterial. The carbon fiber material can be in close proximity to thespray nozzles, such as within about 1 centimeter of the spray nozzles,to receive the plasma. In some embodiments, heaters are disposed abovethe carbon fiber material at the plasma sprayers to maintain theelevated temperature of the carbon fiber material.

Another configuration for continuous carbon nanotube synthesis involvesa special rectangular reactor for the synthesis and growth of carbonnanotubes directly on carbon fiber materials. The reactor can bedesigned for use in a continuous in-line process for producingcarbon-nanotube bearing fibers. In some embodiments, CNTs are grown viaa chemical vapor deposition (“CVD”) process at atmospheric pressure andat elevated temperature in the range of about 550° C. to about 800° C.in a multi-zone reactor. The fact that the synthesis occurs atatmospheric pressure is one factor that facilitates the incorporation ofthe reactor into a continuous processing line for CNT-on-fibersynthesis. Another advantage consistent with in-line continuousprocessing using such a zone reactor is that CNT growth occurs in aseconds, as opposed to minutes (or longer) as in other procedures andapparatus configurations typical in the art.

CNT synthesis reactors in accordance with the various embodimentsinclude the following features:

Rectangular Configured Synthesis Reactors: The cross section of atypical CNT synthesis reactor known in the art is circular. There are anumber of reasons for this including, for example, historical reasons(cylindrical reactors are often used in laboratories) and convenience(flow dynamics are easy to model in cylindrical reactors, heater systemsreadily accept circular tubes (quartz, etc.), and ease of manufacturing.Departing from the cylindrical convention, the present inventionprovides a CNT synthesis reactor having a rectangular cross section. Thereasons for the departure are as follows: 1. Since many carbon fibermaterials that can be processed by the reactor are relatively planarsuch as flat tape or sheet-like in form, a circular cross section is aninefficient use of the reactor volume. This inefficiency results inseveral drawbacks for cylindrical CNT synthesis reactors including, forexample, a) maintaining a sufficient system purge; increased reactorvolume requires increased gas flow rates to maintain the same level ofgas purge. This results in a system that is inefficient for high volumeproduction of CNTs in an open environment; b) increased carbon feedstockgas flow; the relative increase in inert gas flow, as per a) above,requires increased carbon feedstock gas flows. Consider that the volumeof a 12K carbon fiber tow is 2000 times less than the total volume of asynthesis reactor having a rectangular cross section. In an equivalentgrowth cylindrical reactor (i.e., a cylindrical reactor that has a widththat accommodates the same planarized carbon fiber material as therectangular cross-section reactor), the volume of the carbon fibermaterial is 17,500 times less than the volume of the chamber. Althoughgas deposition processes, such as CVD, are typically governed bypressure and temperature alone, volume has a significant impact on theefficiency of deposition. With a rectangular reactor there is a stillexcess volume. This excess volume facilitates unwanted reactions; yet acylindrical reactor has about eight times that volume. Due to thisgreater opportunity for competing reactions to occur, the desiredreactions effectively occur more slowly in a cylindrical reactorchamber. Such a slow down in CNT growth, is problematic for thedevelopment of a continuous process. One benefit of a rectangularreactor configuration is that the reactor volume can be decreased byusing a small height for the rectangular chamber to make this volumeratio better and reactions more efficient. In some embodiments of thepresent invention, the total volume of a rectangular synthesis reactoris no more than about 3000 times greater than the total volume of acarbon fiber material being passed through the synthesis reactor. Insome further embodiments, the total volume of the rectangular synthesisreactor is no more than about 4000 times greater than the total volumeof the carbon fiber material being passed through the synthesis reactor.In some still further embodiments, the total volume of the rectangularsynthesis reactor is less than about 10,000 times greater than the totalvolume of the carbon fiber material being passed through the synthesisreactor. Additionally, it is notable that when using a cylindricalreactor, more carbon feedstock gas is required to provide the same flowpercent as compared to reactors having a rectangular cross section. Itshould be appreciated that in some other embodiments, the synthesisreactor has a cross section that is described by polygonal forms thatare not rectangular, but are relatively similar thereto and provide asimilar reduction in reactor volume relative to a reactor having acircular cross section; c) problematic temperature distribution; when arelatively small-diameter reactor is used, the temperature gradient fromthe center of the chamber to the walls thereof is minimal. But withincreased size, such as would be used for commercial-scale production,the temperature gradient increases. Such temperature gradients result inproduct quality variations across a carbon fiber material substrate(i.e., product quality varies as a function of radial position). Thisproblem is substantially avoided when using a reactor having arectangular cross section. In particular, when a planar substrate isused, reactor height can be maintained constant as the size of thesubstrate scales upward. Temperature gradients between the top andbottom of the reactor are essentially negligible and, as a consequence,thermal issues and the product-quality variations that result areavoided. 2. Gas introduction: Because tubular furnaces are normallyemployed in the art, typical CNT synthesis reactors introduce gas at oneend and draw it through the reactor to the other end. In someembodiments disclosed herein, gas can be introduced at the center of thereactor or within a target growth zone, symmetrically, either throughthe sides or through the top and bottom plates of the reactor. Thisimproves the overall CNT growth rate because the incoming feedstock gasis continuously replenishing at the hottest portion of the system, whichis where CNT growth is most active. This constant gas replenishment isan important aspect to the increased growth rate exhibited by therectangular CNT reactors.

Zoning. Chambers that provide a relatively cool purge zone depend fromboth ends of the rectangular synthesis reactor. Applicants havedetermined that if hot gas were to mix with the external environment(i.e., outside of the reactor), there would be an increase indegradation of the carbon fiber material. The cool purge zones provide abuffer between the internal system and external environments. TypicalCNT synthesis reactor configurations known in the art typically requirethat the substrate is carefully (and slowly) cooled. The cool purge zoneat the exit of the present rectangular CNT growth reactor achieves thecooling in a short period of time, as required for the continuousin-line processing.

Non-contact, hot-walled, metallic reactor. In some embodiments, ahot-walled reactor is made of metal is employed, in particular stainlesssteel. This may appear counterintuitive because metal, and stainlesssteel in particular, is more susceptible to carbon deposition (i.e.,soot and by-product formation). Thus, most CNT reactor configurationsuse quartz reactors because there is less carbon deposited, quartz iseasier to clean, and quartz facilitates sample observation. However,Applicants have observed that the increased soot and carbon depositionon stainless steel results in more consistent, faster, more efficient,and more stable CNT growth. Without being bound by theory it has beenindicated that, in conjunction with atmospheric operation, the CVDprocess occurring in the reactor is diffusion limited. That is, thecatalyst is “overfed;” too much carbon is available in the reactorsystem due to its relatively higher partial pressure (than if thereactor was operating under partial vacuum). As a consequence, in anopen system—especially a clean one—too much carbon can adhere tocatalyst particles, compromising their ability to synthesize CNTs. Insome embodiments, the rectangular reactor is intentionally run when thereactor is “dirty,” that is with soot deposited on the metallic reactorwalls. Once carbon deposits to a monolayer on the walls of the reactor,carbon will readily deposit over itself. Since some of the availablecarbon is “withdrawn” due to this mechanism, the remaining carbonfeedstock, in the form of radicals, react with the catalyst at a ratethat does not poison the catalyst. Existing systems run “cleanly” which,if they were open for continuous processing, would produced a much loweryield of CNTs at reduced growth rates.

Although it is generally beneficial to perform CNT synthesis “dirty” asdescribed above, certain portions of the apparatus, such as gasmanifolds and inlets, can nonetheless negatively impact the CNT growthprocess when soot created blockages. In order to combat this problem,such areas of the CNT growth reaction chamber can be protected with sootinhibiting coatings such as silica, alumina, or MgO. In practice, theseportions of the apparatus can be dip-coated in these soot inhibitingcoatings. Metals such as INVAR® can be used with these coatings as INVARhas a similar CTE (coefficient of thermal expansion) ensuring properadhesion of the coating at higher temperatures, preventing the soot fromsignificantly building up in critical zones.

Combined Catalyst Reduction and CNT Synthesis. In the CNT synthesisreactor disclosed herein, both catalyst reduction and CNT growth occurwithin the reactor. This is significant because the reduction stepcannot be accomplished timely enough for use in a continuous process ifperformed as a discrete operation. In a typical process known in theart, a reduction step typically takes 1-12 hours to perform. Bothoperations occur in a reactor in accordance with the present inventiondue, at least in part, to the fact that carbon feedstock gas isintroduced at the center of the reactor, not the end as would be typicalin the art using cylindrical reactors. The reduction process occurs asthe fibers enter the heated zone; by this point, the gas has had time toreact with the walls and cool off prior to reacting with the catalystand causing the oxidation reduction (via hydrogen radical interactions).It is this transition region where the reduction occurs. At the hottestisothermal zone in the system, the CNT growth occurs, with the greatestgrowth rate occurring proximal to the gas inlets near the center of thereactor.

In some embodiments, when loosely affiliated carbon fiber materials,such as carbon tow are employed, the continuous process can includesteps that spreads out the strands and/or filaments of the tow. Thus, asa tow is unspooled it can be spread using a vacuum-based fiber spreadingsystem, for example. When employing sized carbon fibers, which can berelatively stiff, additional heating can be employed in order to“soften” the tow to facilitate fiber spreading. The spread fibers whichcomprise individual filaments can be spread apart sufficiently to exposean entire surface area of the filaments, thus allowing the tow to moreefficiently react in subsequent process steps. Such spreading canapproach between about 4 inches to about 6 inches across for a 3 k tow.The spread carbon tow can pass through a surface treatment step that iscomposed of a plasma system as described above. After a barrier coatingis applied and roughened, spread fibers then can pass through aCNT-forming catalyst dip bath. The result is fibers of the carbon towthat have catalyst particles distributed radially on their surface. Thecatalyzed-laden fibers of the tow then enter an appropriate CNT growthchamber, such as the rectangular chamber described above, where a flowthrough atmospheric pressure CVD or PE-CVD process is used to synthesizethe CNTs at rates as high as several microns per second. The fibers ofthe tow, now with radially aligned CNTs, exit the CNT growth reactor.

In some embodiments, CNT-infused carbon fiber materials can pass throughyet another treatment process that, in some embodiments is a plasmaprocess used to functionalize the CNTs. Additional functionalization ofCNTs can be used to promote their adhesion to particular resins. Thus,in some embodiments, the present invention provides CNT-infused carbonfiber materials having functionalized CNTs.

As part of the continuous processing of spoolable carbon fibermaterials, the a CNT-infused carbon fiber material can further passthrough a sizing dip bath to apply any additional sizing agents whichcan be beneficial in a final product. Finally if wet winding is desired,the CNT-infused carbon fiber materials can be passed through a resinbath and wound on a mandrel or spool. The resulting carbon fibermaterial/resin combination locks the CNTs on the carbon fiber materialallowing for easier handling and composite fabrication. In someembodiments, CNT infusion is used to provide improved filament winding.Thus, CNTs formed on carbon fibers such as carbon tow, are passedthrough a resin bath to produce resin-impregnated, CNT-infused carbontow. After resin impregnation, the carbon tow can be positioned on thesurface of a rotating mandrel by a delivery head. The tow can then bewound onto the mandrel in a precise geometric pattern in known fashion.

The winding process described above provides pipes, tubes, or otherforms as are characteristically produced via a male mold. But the formsmade from the winding process disclosed herein differ from thoseproduced via conventional filament winding processes. Specifically, inthe process disclosed herein, the forms are made from compositematerials that include CNT-infused tow. Such forms will thereforebenefit from enhanced strength and the like, as provided by theCNT-infused tow.

In some embodiments, a continuous process for infusion of CNTs onspoolable carbon fiber materials can achieve a linespeed between about0.5 ft/min to about 36 ft/min. In this embodiment where the CNT growthchamber is 3 feet long and operating at a 750° C. growth temperature,the process can be run with a linespeed of about 6 ft/min to about 36ft/min to produce, for example, CNTs having a length between about 100nanometers to about 10 microns. The process can also be run with alinespeed of about 1 ft/min to about 6 ft/min to produce, for example,CNTs having a length between about 10 microns to about 100 microns. Theprocess can be run with a linespeed of about 0.5 ft/min to about 1ft/min to produce, for example, CNTs having a length between about 100microns to about 200 microns. The CNT length is not tied only tolinespeed and growth temperature, however, the flow rate of both thecarbon feedstock and the inert carrier gases can also influence CNTlength. For example, a flow rate consisting of less than 1% carbonfeedstock in inert gas at high linespeeds (6 ft/min to 36 ft/min) willresult in CNTs having a length between 1 micron to about 5 microns. Aflow rate consisting of more than 1% carbon feedstock in inert gas athigh linespeeds (6 ft/min to 36 ft/min) will result in CNTs havinglength between 5 microns to about 10 microns.

In some embodiments, more than one carbon material can be runsimultaneously through the process. For example, multiple tapes tows,filaments, strand and the like can be run through the process inparallel. Thus, any number of pre-fabricated spools of carbon fibermaterial can be run in parallel through the process and re-spooled atthe end of the process. The number of spooled carbon fiber materialsthat can be run in parallel can include one, two, three, four, five,six, up to any number that can be accommodated by the width of theCNT-growth reaction chamber. Moreover, when multiple carbon fibermaterials are run through the process, the number of collection spoolscan be less than the number of spools at the start of the process. Insuch embodiments, carbon strands, tows, or the like can be sent througha further process of combining such carbon fiber materials into higherordered carbon fiber materials such as woven fabrics or the like. Thecontinuous process can also incorporate a post processing chopper thatfacilitates the formation CNT-infused chopped fiber mats, for example.

Processes of the invention for CNT infusion to fiber materials allowcontrol of the CNT lengths with uniformity and in a continuous processallowing spoolable fiber materials to be functionalized with CNTs athigh rates. With material residence times between 5 to 300 seconds,linespeeds in a continuous process for a system that is 3 feet long canbe in a range anywhere from about 0.5 ft/min to about 36 ft/min andgreater. The speed selected depends on various parameters as explainedfurther below.

In some embodiments, a material residence time of about 5 to about 30seconds can produce CNTs having a length between about 100 nanometers toabout 10 microns. In some embodiments, a material residence time ofabout 30 to about 180 seconds can produce CNTs having a length betweenabout 10 microns to about 100 microns. In still further embodiments, amaterial residence time of about 180 to about 300 seconds can produceCNTs having a length between about 100 microns to about 500 microns. Oneskilled in the art will recognize that these ranges are approximate andthat CNT length can also be modulated by reaction temperatures, andcarrier and carbon feedstock concentrations and flow rates.

EXAMPLE

This example shows how a carbon fiber material can be infused with CNTsin a continuous process to enhance signature control capabilities incomposite structures.

In this example, the maximum loading of CNTs on fibers is targeted.34-700 12 k carbon fiber tow with a tex value of 800 (Grafil Inc.,Sacramento, Calif.) is implemented as the carbon fiber substrate. Theindividual filaments in this carbon fiber tow have a diameter ofapproximately 7 μm.

FIG. 12 depicts system 800 for producing CNT-infused fiber in accordancewith the illustrative embodiment of the present invention. System 800includes a carbon fiber material payout and tensioner station 805,sizing removal and fiber spreader station 810, plasma treatment station815, barrier coating application station 820, air dry station 825,catalyst application station 830, solvent flash-off station 835,CNT-infusion station 840, fiber bundler station 845, and carbon fibermaterial uptake bobbin 850, interrelated as shown.

Payout and tension station 805 includes payout bobbin 806 and tensioner807. The payout bobbin delivers carbon fiber material 860 to theprocess; the fiber is tensioned via tensioner 807. For this example, thecarbon fiber is processed at a linespeed of 2 ft/min.

Fiber material 860 is delivered to sizing removal and fiber spreaderstation 810 which includes sizing removal heaters 865 and fiber spreader870. At this station, any “sizing” that is on fiber 860 is removed.Typically, removal is accomplished by burning the sizing off of thefiber. Any of a variety of heating means can be used for this purpose,including, for example, an infrared heater, a muffle furnace, and othernon-contact heating processes. Sizing removal can also be accomplishedchemically. The fiber spreader separates the individual elements of thefiber. Various techniques and apparatuses can be used to spread fiber,such as pulling the fiber over and under flat, uniform-diameter bars, orover and under variable-diameter bars, or over bars withradially-expanding grooves and a kneading roller, over a vibratory bar,etc. Spreading the fiber enhances the effectiveness of downstreamoperations, such as plasma application, barrier coating application, andcatalyst application, by exposing more fiber surface area.

Multiple sizing removal heaters 865 can be placed throughout the fiberspreader 870 which allows for gradual, simultaneous desizing andspreading of the fibers. Payout and tension station 805 and sizingremoval and fiber spreader station 810 are routinely used in the fiberindustry; those skilled in the art will be familiar with their designand use.

The temperature and time required for burning off the sizing vary as afunction of (1) the sizing material and (2) the commercialsource/identity of carbon fiber material 860. A conventional sizing on acarbon fiber material can be removed at about 650° C. At thistemperature, it can take as long as 15 minutes to ensure a complete burnoff of the sizing. Increasing the temperature above this burntemperature can reduce burn-off time. Thermogravimetric analysis is usedto determine minimum burn-off temperature for sizing for a particularcommercial product.

Depending on the timing required for sizing removal, sizing removalheaters may not necessarily be included in the CNT-infusion processproper; rather, removal can be performed separately (e.g., in parallel,etc.). In this way, an inventory of sizing-free carbon fiber materialcan be accumulated and spooled for use in a CNT-infused fiber productionline that does not include fiber removal heaters. The sizing-free fiberis then spooled in payout and tension station 805. This production linecan be operated at higher speed than one that includes sizing removal.

Unsized fiber 880 is delivered to plasma treatment station 815. For thisexample, atmospheric plasma treatment is utilized in a ‘downstream’manner from a distance of 1 mm from the spread carbon fiber material.The gaseous feedstock is comprised of 100% helium.

Plasma enhanced fiber 885 is delivered to barrier coating station 820.In this illustrative example, a siloxane-based barrier coating solutionis employed in a dip coating configuration. The solution is ‘AccuglassT-11 Spin-On Glass’ (Honeywell International Inc., Morristown, N.J.)diluted in isopropyl alcohol by a dilution rate of 40 to 1 by volume.The resulting barrier coating thickness on the carbon fiber material isapproximately 40 nm. The barrier coating can be applied at roomtemperature in the ambient environment.

Barrier coated carbon fiber 890 is delivered to air dry station 825 forpartial curing of the nanoscale barrier coating. The air dry stationsends a stream of heated air across the entire carbon fiber spread.Temperatures employed can be in the range of 100° C. to about 500° C.

After air drying, barrier coated carbon fiber 890 is delivered tocatalyst application station 830. In this example, an iron oxide-basedCNT forming catalyst solution is employed in a dip coatingconfiguration. The solution is ‘EFH-1’ (Ferrotec Corporation, Bedford,N.H.) diluted in hexane by a dilution rate of 200 to 1 by volume. At theprocess linespeed for CNT-infused fiber targeted at enhancing signaturecontrol characteristics, the fiber will remain in the dip bath for 10seconds. The catalyst can be applied at room temperature in the ambientenvironment with neither vacuum nor an inert atmosphere required. Amonolayer of catalyst coating is achieved on the carbon fiber material.‘EFH-1’ prior to dilution has a nanoparticle concentration ranging from3-15% by volume. The iron oxide nanoparticles are of composition Fe₂O₃and Fe₃O₄ and are approximately 8 nm in diameter.

Catalyst-laden carbon fiber material 895 is delivered to solventflash-off station 835. The solvent flash-off station sends a stream ofair across the entire carbon fiber spread. In this example, roomtemperature air can be employed in order to flash-off all hexane left onthe catalyst-laden carbon fiber material.

After solvent flash-off, catalyst-laden fiber 895 is finally advanced toCNT-infusion station 840. In this example, a rectangular reactor with a12 inch growth zone is used to employ CVD growth at atmosphericpressure. 97.6% of the total gas flow is inert gas (Nitrogen) and theother 2.4% is the carbon feedstock (acetylene). The growth zone is heldat 750° C. For the rectangular reactor mentioned above, 750° C. is arelatively high growth temperature, which allows for the highest growthrates possible. Catalyst laden fibers are exposed to the CNT growthenvironment for 30 seconds in this example, resulting in 60 micron longwith approximately 4% volume percentage CNTs infused to the carbon fibersurface.

After CNT-infusion, CNT-infused fiber 897 is re-bundled at fiber bundlerstation 845. This operation recombines the individual strands of thefiber, effectively reversing the spreading operation that was conductedat station 810.

The bundled, CNT-infused fiber 897 is wound about uptake fiber bobbin850 for storage. CNT-infused fiber 897 is then ready for use in avariety of applications which require enhanced signature controlcharacteristics. In this case, this material is dry wound and resininfused to act as the back layer of a RAM panel as shown in FIG. 13. Theresulting panel layer of the above described procedure has between 2-4%CNTs by weight in the composite structure.

It is noteworthy that some of the operations described above can beconducted under inert atmosphere or vacuum for environmental isolation.For example, if sizing is being burned off of a carbon fiber material,the fiber can be environmentally isolated to contain off-gassing andprevent damage from moisture. For convenience, in system 800,environmental isolation is provided for all operations, with theexception of carbon fiber material payout and tensioning, at thebeginning of the production line, and fiber uptake, at the end of theproduction line.

It is to be understood that the above-described embodiments are merelyillustrative of the present invention and that many variations of theabove-described embodiments can be devised by those skilled in the artwithout departing from the scope of the invention. For example, in thisSpecification, numerous specific details are provided in order toprovide a thorough description and understanding of the illustrativeembodiments of the present invention. Those skilled in the art willrecognize, however, that the invention can be practiced without one ormore of those details, or with other processes, materials, components,etc.

Furthermore, in some instances, well-known structures, materials, oroperations are not shown or described in detail to avoid obscuringaspects of the illustrative embodiments. It is understood that thevarious embodiments shown in the Figures are illustrative, and are notnecessarily drawn to scale. Reference throughout the specification to“one embodiment” or “an embodiment” or “some embodiments” means that aparticular feature, structure, material, or characteristic described inconnection with the embodiment(s) is included in at least one embodimentof the present invention, but not necessarily all embodiments.Consequently, the appearances of the phrase “in one embodiment,” “in anembodiment,” or “in some embodiments” in various places throughout theSpecification are not necessarily all referring to the same embodiment.Furthermore, the particular features, structures, materials, orcharacteristics can be combined in any suitable manner in one or moreembodiments. It is therefore intended that such variations be includedwithin the scope of the following claims and their equivalents.

1. A radar absorbing composite comprising a (CNT)-infused fiber materialdisposed in at least a portion of a matrix material, said compositebeing capable of absorbing radar in a frequency range from between about0.10 Megahertz to about 60 Gigahertz, said CNT-infused fiber materialforming a first layer that reduces radar reflectance and a second layerthat dissipates the energy of the absorbed radar.
 2. The composite ofclaim 1 further comprising a plurality of transition metalnanoparticles.
 3. The composite of claim 2, wherein said nanoparticlescomprise iron.
 4. The composite of claim 1, further comprising aplurality of additional layers between said first layer and said secondlayer.
 5. The composite of claim 4, wherein said plurality of additionallayers comprises a stepped gradient of increasing CNT density on saidCNT-infused fiber material from said first layer down to said secondlayer.
 6. The composite of claim 4, wherein said plurality of additionallayers comprises a continuous gradient of increasing CNT density on saidCNT-infused fiber material from said first layer down to said secondlayer.
 7. The composite of claim 1, wherein said first layer and saidsecond layer comprise separate CNT-infused fiber materials.
 8. Thecomposite of claims 7, wherein said first layer comprises a CNT-infusedglass fiber material.
 9. The composite of claim 7, wherein said secondlayer comprises a CNT-infused carbon fiber material.
 10. The compositeof claim 1, wherein the CNTs are present in a range between about 0.001%by weight to about 20% by weight of the composite.
 11. The compositematerial of claim 1, wherein said CNT-infused fiber material comprises afiber material selected from glass, carbon, and ceramic.
 12. Thecomposite material of claim 1, wherein the CNTs infused on the fibermaterial have a controlled orientation within the composite.
 13. Amethod of manufacturing a radar absorbing composite, said compositecomprising a (CNT)-infused fiber material disposed in at least a portionof a matrix material, said composite being capable of absorbing radar ina frequency range from between about 0.10 Megahertz to about 60Gigahertz, said CNT-infused fiber material forming a first layer thatreduces radar reflectance and a second layer that dissipates the energyof the absorbed radar, the method comprising disposing a CNT-infusedfiber material in a portion of a matrix material with a controlledorientation of the CNT-infused fiber material within the matrixmaterial, and curing the matrix material, wherein the controlledorientation of the CNT-infused fiber material controls the relativeorientation of CNTs infused thereon.
 14. A panel comprising a composite,said composite comprising a (CNT)-infused fiber material disposed in atleast a portion of a matrix material, said composite being capable ofabsorbing radar in a frequency range from between about 0.10 Megahertzto about 60 Gigahertz, said CNT-infused fiber material forming a firstlayer that reduces radar reflectance and a second layer that dissipatesthe energy of the absorbed radar, said panel being adaptable as astructural component of a transport vessel or missile for use in stealthapplications.
 15. A transport vessel comprising the panel of claim 14,wherein the CNTs infused on the fiber material have a controlledorientation within the composite material.
 16. The transport vessel ofclaim 15, wherein said transport vessel is selected from a boat, aplane, and a ground vehicle.
 17. A projectile comprising the panel ofclaim 14, wherein the CNTs infused on the fiber material have acontrolled orientation within the composite material.